### What
This PR's goal is to improve the throughput performance of the Next.js
server when handling Pages Router routes.
Note that the results from this are very synthetic and do not represent
the real-life performance of an application. If we only wanted to handle
hello worlds, we could probably make this even faster but on production,
a slow fetch call to your DB is probably what's slowing you down.
I'll look into App Router next.
### Why?
I guess I got nerd-sniped into it 😃
### How?
A few optimizations:
- I looked deeply at the pipeline for rendering a Pages Router page. I
noticed a lot of intermediary streams being created here and there to
eventually be concatenated to a simple string. I think this is probably
left over code from when we wanted to support streaming there and so
there's some code that was shared with the App Router, which we
absolutely don't need I think. I refactored it to be slightly simpler
with just a few string concats here and there.
- misc: I removed some redundant Promises being created here and there
and added a small inline optimisation to eliminate `if (renderOpts.dev)`
code in production.
### Nummies
Test setup: hello world pages router app, next start + autocannon
- requests handled in 10s: 18k -> 33K, **~80% improvement**
- avg latency: 4.89ms -> 2.8ms, **~42% improvement**
- avg req/res: 1846.5 -> 2983.5, **~61% improvement**
Before
<img width="742" alt="image"
src="https://github.com/vercel/next.js/assets/11064311/658e7ade-eba7-4604-a7c9-619bd51a5ec8">
vs
after
<img width="880" alt="image"
src="https://github.com/vercel/next.js/assets/11064311/2f46cf69-b788-4db2-bf90-6f65dc7abd82">
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Closes NEXT-3103
d52d32f423