Vercel dev + Vite is incredibly slow; latency added to every proxy req

[izhan]

Vercel dev + Vite is incredibly slow; latency added to every proxy req

[izhan]

Had some time to find the root cause:

1. We make a call to getVercelConfig() with every request here: https://github.com/vercel/vercel/blob/main/packages/cli/src/util/dev/server.ts#L1284
2. That call is slow when config.builds is not specified (which is usually blank for most projects). Specifically this code block here is the bottleneck: https://github.com/vercel/vercel/blob/main/packages/cli/src/util/dev/server.ts#L579. I'm guessing it's because there are I/O events w/ detectBuilders
3. This means every file request will wait for the config file to be loaded before completing (50ms on my machine). This should be fine, but fetching config files is batched, but not cached. Chrome has a max connection of 6 files, meaning after the initial 6 files are complete, another round of 6 files are fetched again, and the whole config file lookup starts over again.

The only workaround I could find right now is to specify the builds option in config, so that no I/O events are even needed. Unfortunately just found another bug that prevents it from working :( #10862