Significant TTFB delay on Cloudflare Worker vs Vercel using same OpenNext setup

I'm using OpenNext to deploy a Next.js 15.3 site with ISR and streaming enabled.

When deployed to:

Vercel: the initial document load (document in Network tab) is ~389ms

Cloudflare Workers: same route takes ~6.3s for the initial document response

Both environments use the exact same codebase, and both are rendering an ISR page (non-cached). The rest of the assets load quickly in both cases. Only the HTML response (document) is delayed on Cloudflare.
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