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App Router: Chunk <Script> Tags Get No Fetchpriority And Chrome'S LowPriorityAsyncScriptExecution Delays Their Evaluation — Hydration Render Storm (~30K Commits) On Cross-Site AssetPrefix CDNs

Next.js App Router emits client/page chunks as async scripts without fetchpriority, making them subject to Chrome's LowPriorityAsyncScriptExecution intervention. On cross-site CDNs, this delays chunk evaluation, causing React hydration to enter a retry storm with ~30,000 commits, saturating the main thread.

highConfidence 85%Next.js Affected V15.5.18

Origin Analysis

Next.js fails to set fetchpriority on parser-inserted async chunk scripts, while React's hydration loop does not quiet-wait on suspended chunks, instead retrying aggressively. The underlying design flaw is the absence of config plumbing for fetchpriority and inadequate suspension handling in React's vendored canary.

1. Build and start a Next.js app with cross-site assetPrefix (or force Chrome flag LowPriorityAsyncScriptExecution:low_pri_async_exec_cross_site_only/false). 2. In Chrome DevTools Performance, enable CPU 6x slowdown. 3. Cold-navigate to the page and measure loadEventStart - domContentLoadedEventEnd. 4. Compare with --disable-features=LowPriorityAsyncScriptExecution to see the delay.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This workaround requires a custom server, bypassing `next start`, and its regex may miss dynamically injected scripts or match non-chunk scripts, potentially altering priority of unrelated resources. Upgrading Next.js could change HTML structure, breaking the regex. Rollback suggestion: remove the custom server and restore `next start` once Next.js officially supports scriptFetchPriority or the issue is resolved. Additionally, setting high priority on all chunks may increase network contention; monitor performance.