diff --git a/src/outputs/writers.ts b/src/outputs/writers.ts
index 1b41376..246c173 100644
--- a/src/outputs/writers.ts
+++ b/src/outputs/writers.ts
@@ -1,5 +1,6 @@
 import * as fs from "node:fs/promises";
 import * as path from "node:path";
+import * as os from "node:os";
 import {
   ContentItem,
   Thread,
@@ -17,33 +18,66 @@ async function ensureDir(p: string) {
   await fs.mkdir(p, { recursive: true });
 }
 
+const DEFAULT_COPY_CONCURRENCY = Math.max(
+  2,
+  Math.min(
+    32,
+    typeof os.availableParallelism === "function"
+      ? os.availableParallelism()
+      : 8
+  )
+);
+
 /**
  * Copy media attachments for a set of items into imagesDir, prefixing names with "_".
  * If an attachment lacks absPath, it will be skipped with a warning.
+ * Copies are performed with bounded concurrency to speed up large archives.
  */
 async function copyMedia(
   items: ContentItem[],
   imagesDir: string,
-  logger: (l: Level, m: string) => void,
+  logger: (l: Level, m: string) => void
 ) {
   await ensureDir(imagesDir);
+
+  const copies: Array<{ src: string; dest: string }> = [];
   for (const it of items) {
     for (const m of it.media ?? []) {
-      const base = m.absPath ? path.basename(m.absPath) : `${m.id}.bin`;
+      if (!m.absPath) {
+        logger("warn", `No absPath for media ${m.id}; skipping copy`);
+        continue;
+      }
+      const base = path.basename(m.absPath);
+      copies.push({ src: m.absPath, dest: path.join(imagesDir, `_${base}`) });
+    }
+  }
+
+  if (!copies.length) return;
+
+  const parsedEnv = Number.parseInt(
+    process.env.SPLICE_MEDIA_CONCURRENCY ?? "",
+    10
+  );
+  const concurrency =
+    Number.isFinite(parsedEnv) && parsedEnv > 0
+      ? parsedEnv
+      : DEFAULT_COPY_CONCURRENCY;
+
+  let next = 0;
+  const worker = async () => {
+    while (true) {
+      const idx = next++;
+      if (idx >= copies.length) break;
+      const { src, dest } = copies[idx];
       try {
-        if (!m.absPath) {
-          logger("warn", `No absPath for media ${m.id}; skipping copy`);
-          continue;
-        }
-        await fs.copyFile(m.absPath, path.join(imagesDir, `_${base}`));
+        await fs.copyFile(src, dest);
       } catch (e) {
-        logger(
-          "warn",
-          `Failed to copy media ${m.absPath ?? m.id}: ${(e as Error).message}`,
-        );
+        logger("warn", `Failed to copy media ${src}: ${(e as Error).message}`);
       }
     }
-  }
+  };
+
+  await Promise.all(Array.from({ length: concurrency }, worker));
 }
 
 /**
@@ -83,7 +117,7 @@ function isolateQuotedTweetLinks(text: string): string {
 
 /**
  * Write Markdown outputs:
- * - threads/&lt;yyyymmdd&gt;-thread-&lt;slug&gt;.md with frontmatter, cleaned text, media links, and link to Twitter
+ * - threads/&lt;yyyymmdd&gt;/&lt;slug&gt;.md with frontmatter, cleaned text, media links, and link to Twitter
  * - tweets/&lt;yyyymmdd&gt;-tweet-&lt;slug&gt;.md for non-thread tweets (excluding RTs)
  * - images/_&lt;file&gt; copied for referenced items
  */
@@ -142,7 +176,9 @@ export async function writeMarkdown(
     const ymd = date.replace(/-/g, "");
     const filePath = path.join(threadsDir, `${ymd}/${name}.md`);
     const topLink = `https://twitter.com/i/web/status/${first.id}`;
-    const body = `${fm}\n${parts.join("\n\n")}\n\n[View on Twitter](${topLink})`;
+    const body = `${fm}\n${parts.join(
+      "\n\n",
+    )}\n\n[View on Twitter](${topLink})`;
 
     if (dryRun) {
       logger("info", `(dry-run) would write thread file: ${filePath}`);