Workerd is already open source so that's a good start.

It surely depends on your use case. Testing my Ricochet Robots solver (https://ricochetrobots.kevincox.ca/) which is pure computation with effectively no IO the speed is basically indistinguishable. Some runs the WASM is faster sometimes the native is faster. On average the native is definitely faster but it is surprisingly within the noise.

Last time I compared (about 8 years ago) WASM was closer to double the runtime. So things have definitely improved. (I had to check a handful of times that I was compiling with the correct optimizations in both cases.)

The stats I've seen show a 10-20% loss in speed relative to natively-compiled, which is effectively noise for all but the most critical paths.

It may get even closer with WASM3, released 2 months ago, since it has things like 64 bit address support, more flexible vector instructions, typed references (which remove runtime safety checks), basic GC, etc. https://webassembly.org/news/2025-09-17-wasm-3.0/

> Also think the time for a FOSS alternative is coming. Serverless without, virtually, cold starts is here to stay but being tied to only 1 vendor is problematic.

Supabase Edge Functions runs on the same V8 isolate primitive as Cloudflare Workers and is fully open-source (https://github.com/supabase/edge-runtime). We use the Deno runtime, which supports Node built-in APIs, npm packages, and WebAssembly (WASM) modules. (disclaimer: I'm the lead for Supabase Edge Functions)

Workers is a v8 isolates runtime like Deno. v8 and Deno are both open source and Deno is used in a variety of platforms, including Supabase and ValTown.

It is a terrific technology, and it is reasonably portable but I think you would be better using it in something like Supabase where are the whole platform is open source and portable, if those are your goals.