diff --git a/src-tauri/src/task.rs b/src-tauri/src/task.rs
index e876880..48e54e9 100644
--- a/src-tauri/src/task.rs
+++ b/src-tauri/src/task.rs
@@ -265,7 +265,13 @@ async fn run_pipeline(
         },
         |segment| {
             if let Some(ref tx) = seg_tx_for_callback {
-                let _ = tx.try_send(segment.clone());
+                let tx = tx.clone();
+                let seg = segment.clone();
+                tauri::async_runtime::spawn(async move {
+                    if let Err(e) = tx.send(seg).await {
+                        eprintln!("translation: send error: {e}");
+                    }
+                });
             }
             if let Ok(mut current_task) = app_state_for_segment.get_task(&task_id_for_segment) {
                 upsert_segment(&mut current_task.segments, segment.clone());
@@ -310,7 +316,7 @@ async fn incremental_translate(
     task_id: &str,
     target_lang: &TargetLanguage,
 ) -> Result<()> {
-    let batch_size = translator.batch_size().clamp(10, 15);
+    let batch_size = translator.batch_size().clamp(3, 60);
     let context_size = translator.context_size().min(5);
     let mut all_segments: Vec<SubtitleSegment> = Vec::new();
     let mut buffer: Vec<SubtitleSegment> = Vec::new();
diff --git a/src-tauri/src/translate.rs b/src-tauri/src/translate.rs
index 306a202..6f9a56c 100644
--- a/src-tauri/src/translate.rs
+++ b/src-tauri/src/translate.rs
@@ -91,7 +91,7 @@ impl Translator {
         PF: FnMut(f32),
         SF: FnMut(SubtitleSegment),
     {
-        let batch_size = self.config.batch_size.clamp(10, 15);
+        let batch_size = self.config.batch_size.clamp(3, 350);
         let context_size = self.config.context_size.min(5);
         let mut translated = segments.to_vec();
         let target_language_name = match target_language {
diff --git a/src/components/SubtitleEditor.vue b/src/components/SubtitleEditor.vue
index 687f90f..dcca131 100644
--- a/src/components/SubtitleEditor.vue
+++ b/src/components/SubtitleEditor.vue
@@ -1,7 +1,22 @@
 <script setup lang="ts">
-import { computed, ref } from 'vue'
+import { computed, ref, nextTick, watch, onBeforeUpdate } from 'vue'
 import type { SubtitleSegment, SubtitleTask } from '../lib/types'
 
+const LOG_ROW_HEIGHT = 20
+const LOG_OVERSCAN = 10
+
+const SEGMENT_TIMESTAMP_HEIGHT = 20
+const SEGMENT_TEXTAREA_HEIGHT = 60
+const SEGMENT_PADDING_GAP = 34
+const SEGMENT_CHARS_PER_LINE = 40
+const SEGMENT_LINE_HEIGHT = 20
+const SEGMENT_OVERSCAN = 5
+
+function estimateSegmentHeight(seg: SubtitleSegment): number {
+  const textLines = Math.max(1, Math.ceil((seg.sourceText?.length || 1) / SEGMENT_CHARS_PER_LINE))
+  return SEGMENT_TIMESTAMP_HEIGHT + textLines * SEGMENT_LINE_HEIGHT + SEGMENT_TEXTAREA_HEIGHT + SEGMENT_PADDING_GAP
+}
+
 const props = defineProps<{
   task: SubtitleTask | null
   logs: string[]
@@ -30,15 +45,145 @@ const emit = defineEmits<{
   translate: []
 }>()
 
-const segments = computed(() =>
+const sortedSegments = computed(() =>
   [...(props.task?.segments ?? [])].sort((left, right) => {
     if (left.start !== right.start) return left.start - right.start
     if (left.end !== right.end) return left.end - right.end
     return left.id.localeCompare(right.id)
   }),
 )
+
 const logsExpanded = ref(false)
 
+const logContainerRef = ref<HTMLElement | null>(null)
+const logScrollTop = ref(0)
+const isNearBottom = ref(true)
+
+const visibleLogs = computed(() => {
+  const total = props.logs.length
+  if (total === 0) return { items: [] as string[], start: 0, paddingTop: 0, paddingBottom: 0 }
+
+  const containerHeight = logContainerRef.value?.clientHeight ?? 220
+  const startIndex = Math.max(0, Math.floor(logScrollTop.value / LOG_ROW_HEIGHT) - LOG_OVERSCAN)
+  const visibleCount = Math.ceil(containerHeight / LOG_ROW_HEIGHT) + LOG_OVERSCAN * 2
+  const endIndex = Math.min(total, startIndex + visibleCount)
+
+  return {
+    items: props.logs.slice(startIndex, endIndex),
+    start: startIndex,
+    paddingTop: startIndex * LOG_ROW_HEIGHT,
+    paddingBottom: Math.max(0, total * LOG_ROW_HEIGHT - endIndex * LOG_ROW_HEIGHT),
+  }
+})
+
+function handleLogScroll(event: Event) {
+  const el = event.target as HTMLElement
+  logScrollTop.value = el.scrollTop
+  isNearBottom.value = el.scrollTop + el.clientHeight >= el.scrollHeight - 40
+}
+
+watch(() => props.logs.length, () => {
+  if (isNearBottom.value) {
+    nextTick(() => {
+      if (logContainerRef.value) {
+        logContainerRef.value.scrollTop = logContainerRef.value.scrollHeight
+        logScrollTop.value = logContainerRef.value.scrollTop
+      }
+    })
+  }
+})
+
+const segContainerRef = ref<HTMLElement | null>(null)
+const segScrollTop = ref(0)
+const segHeights = ref(new Map<string, number>())
+const segObservers = new Map<string, ResizeObserver>()
+
+const segOffsetCache = computed(() => {
+  const segments = sortedSegments.value
+  const cache = new Map<string, { height: number; offset: number }>()
+  let offset = 0
+  for (const seg of segments) {
+    const height = segHeights.value.get(seg.id) ?? estimateSegmentHeight(seg)
+    cache.set(seg.id, { height, offset })
+    offset += height + 8
+  }
+  return { cache, totalHeight: Math.max(0, offset - 8) }
+})
+
+const visibleSegments = computed(() => {
+  const segments = sortedSegments.value
+  if (segments.length === 0) return { items: [] as SubtitleSegment[], paddingTop: 0, paddingBottom: 0 }
+
+  const containerHeight = segContainerRef.value?.clientHeight ?? 600
+  const { cache, totalHeight } = segOffsetCache.value
+
+  let startIndex = segments.length - 1
+  for (let i = 0; i < segments.length; i++) {
+    const info = cache.get(segments[i].id)!
+    if (info.offset + info.height > segScrollTop.value) {
+      startIndex = Math.max(0, i - SEGMENT_OVERSCAN)
+      break
+    }
+  }
+
+  let endIndex = segments.length
+  for (let i = startIndex; i < segments.length; i++) {
+    const info = cache.get(segments[i].id)!
+    if (info.offset > segScrollTop.value + containerHeight) {
+      endIndex = Math.min(segments.length, i + SEGMENT_OVERSCAN)
+      break
+    }
+  }
+
+  const startOffset = cache.get(segments[startIndex].id)?.offset ?? 0
+  const endInfo = cache.get(segments[Math.min(endIndex - 1, segments.length - 1)].id)
+  const endOffset = endInfo ? endInfo.offset + endInfo.height : totalHeight
+  const paddingBottom = Math.max(0, totalHeight - endOffset)
+
+  return {
+    items: segments.slice(startIndex, endIndex),
+    paddingTop: startOffset,
+    paddingBottom,
+  }
+})
+
+function handleSegScroll(event: Event) {
+  const el = event.target as HTMLElement
+  segScrollTop.value = el.scrollTop
+}
+
+function onSegMount(el: HTMLElement, segmentId: string) {
+  const existing = segObservers.get(segmentId)
+  if (existing) existing.disconnect()
+
+  const ro = new ResizeObserver((entries) => {
+    for (const entry of entries) {
+      const actualHeight = entry.contentBoxSize
+        ? entry.contentBoxSize[0].blockSize
+        : entry.borderBoxSize
+          ? entry.borderBoxSize[0].blockSize
+          : (entry.target as HTMLElement).offsetHeight
+      if (actualHeight > 0) {
+        segHeights.value = new Map(segHeights.value).set(segmentId, actualHeight)
+      }
+    }
+  })
+  ro.observe(el)
+  segObservers.set(segmentId, ro)
+}
+
+function onSegUnmount(segmentId: string) {
+  const ro = segObservers.get(segmentId)
+  if (ro) {
+    ro.disconnect()
+    segObservers.delete(segmentId)
+  }
+}
+
+onBeforeUpdate(() => {
+  segScrollTop.value = segContainerRef.value?.scrollTop ?? 0
+})
+
 function formatTime(seconds: number) {
   const ms = Math.round(seconds * 1000)
   const h = Math.floor(ms / 3600000)
@@ -59,7 +204,7 @@ function updateTranslatedText(segment: SubtitleSegment, value: string) {
       <div>
         <strong>{{ task?.fileName ?? $t('editor.title') }}</strong>
         <p class="panel-subtitle">
-          {{ task ? $t('editor.segments', { count: segments.length }) : $t('editor.selectTask') }}
+          {{ task ? $t('editor.segments', { count: sortedSegments.length }) : $t('editor.selectTask') }}
         </p>
       </div>
       <div v-if="task" class="export-actions">
@@ -80,31 +225,34 @@ function updateTranslatedText(segment: SubtitleSegment, value: string) {
       <p style="margin-top: 6px; font-size: 11px;">{{ $t('editor.emptyHint') }}</p>
     </div>
 
-    <div v-else-if="segments.length === 0" class="empty-state">
+    <div v-else-if="sortedSegments.length === 0" class="empty-state">
       <template v-if="isProcessing">{{ $t('editor.processing') }}</template>
       <template v-else-if="task?.status === 'failed'">{{ $t('editor.failed') }}</template>
       <template v-else>{{ $t('editor.noSegments') }}</template>
     </div>
 
-    <div v-else class="segment-list">
-      <article
-        v-for="segment in segments"
-        :key="segment.id"
-        class="segment-item"
-      >
-        <div class="task-row subtle">
-          <span>{{ formatTime(segment.start) }} - {{ formatTime(segment.end) }}</span>
-          <span class="segment-id">{{ segment.id }}</span>
-        </div>
-        <p class="source-text">{{ segment.sourceText || $t('editor.waiting') }}</p>
-        <textarea
-          class="editor-input"
-          :value="segment.translatedText ?? ''"
-          :placeholder="task.outputMode === 'translate' ? $t('editor.translation') : $t('editor.source')"
-          :disabled="task.outputMode === 'source'"
-          @change="updateTranslatedText(segment, ($event.target as HTMLTextAreaElement).value)"
-        />
-      </article>
+    <div v-else ref="segContainerRef" class="segment-list" @scroll="handleSegScroll">
+      <div class="seg-spacer" :style="{ paddingTop: `${visibleSegments.paddingTop}px`, paddingBottom: `${visibleSegments.paddingBottom}px` }">
+        <article
+          v-for="segment in visibleSegments.items"
+          :key="segment.id"
+          :ref="(el: any) => { if (el) onSegMount(el as HTMLElement, segment.id); else onSegUnmount(segment.id) }"
+          class="segment-item"
+        >
+          <div class="task-row subtle">
+            <span>{{ formatTime(segment.start) }} - {{ formatTime(segment.end) }}</span>
+            <span class="segment-id">{{ segment.id }}</span>
+          </div>
+          <p class="source-text">{{ segment.sourceText || $t('editor.waiting') }}</p>
+          <textarea
+            class="editor-input"
+            :value="segment.translatedText ?? ''"
+            :placeholder="task.outputMode === 'translate' ? $t('editor.translation') : $t('editor.source')"
+            :disabled="task.outputMode === 'source'"
+            @change="updateTranslatedText(segment, ($event.target as HTMLTextAreaElement).value)"
+          />
+        </article>
+      </div>
     </div>
 
     <div class="log-drawer" :class="{ expanded: logsExpanded }">
@@ -117,8 +265,10 @@ function updateTranslatedText(segment: SubtitleSegment, value: string) {
         <div v-if="logs.length === 0" class="empty-state">
           {{ $t('editor.noLogs') }}
         </div>
-        <div v-else class="log-list">
-          <pre v-for="(line, index) in logs" :key="`${index}-${line}`" class="log-line">{{ line }}</pre>
+        <div v-else ref="logContainerRef" class="log-list" @scroll="handleLogScroll">
+          <div class="log-spacer" :style="{ paddingTop: `${visibleLogs.paddingTop}px`, paddingBottom: `${visibleLogs.paddingBottom}px` }">
+            <pre v-for="(line, i) in visibleLogs.items" :key="visibleLogs.start + i" class="log-line">{{ line }}</pre>
+          </div>
         </div>
       </div>
     </div>
diff --git a/src/style.css b/src/style.css
index b2ac904..cbc6d23 100644
--- a/src/style.css
+++ b/src/style.css
@@ -395,6 +395,11 @@ textarea {
   flex: 1 1 auto;
   overflow: auto;
   padding-right: 2px;
+  will-change: scroll-position;
+}
+
+.seg-spacer {
+  min-width: 100%;
 }
 
 .task-item,
@@ -617,6 +622,11 @@ textarea {
   border: 1px solid var(--c-border);
   border-radius: var(--radius-md);
   background: var(--c-log-bg);
+  will-change: scroll-position;
+}
+
+.log-spacer {
+  min-width: 100%;
 }
 
 .log-line {
@@ -627,6 +637,7 @@ textarea {
   line-height: 1.6;
   color: var(--c-log-text);
   font-family: "SF Mono", "Fira Code", "Cascadia Code", monospace;
+  content-visibility: auto;
 }
 
 .log-line + .log-line {
@@ -679,6 +690,7 @@ textarea {
   border-left: 3px solid transparent;
   transition: border-color var(--transition), background var(--transition), box-shadow var(--transition);
   position: relative;
+  content-visibility: auto;
 }
 
 .segment-item:hover {