perf: improve Pages Router server rendering performance

[feedthejim]

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

vs

after

Also ran on the other benchmark by @eknkc. Still can do better!

Before:

After:

Closes NEXT-3103

[ijjk]

Tests Passed

[ijjk]

Stats from current PR

Default Build (Increase detected ⚠️)

General

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
buildDuration	16s	16.1s	N/A
buildDurationCached	8.8s	7.4s	N/A
nodeModulesSize	199 MB	199 MB	N/A
nextStartRea..uration (ms)	432ms	436ms	N/A

Client Bundles (main, webpack)

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
2453-HASH.js gzip	31.4 kB	31.4 kB	N/A
3304.HASH.js gzip	181 B	181 B	✓
3f784ff6-HASH.js gzip	53.7 kB	53.7 kB	✓
8299-HASH.js gzip	5.1 kB	5.1 kB	N/A
framework-HASH.js gzip	45.2 kB	45.2 kB	✓
main-app-HASH.js gzip	242 B	240 B	N/A
main-HASH.js gzip	29.6 kB	29.7 kB	N/A
webpack-HASH.js gzip	1.68 kB	1.68 kB	N/A
Overall change	99 kB	99 kB	✓

Legacy Client Bundles (polyfills)

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
polyfills-HASH.js gzip	31 kB	31 kB	✓
Overall change	31 kB	31 kB	✓

Client Pages

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
_app-HASH.js gzip	196 B	197 B	N/A
_error-HASH.js gzip	184 B	184 B	✓
amp-HASH.js gzip	505 B	505 B	✓
css-HASH.js gzip	324 B	325 B	N/A
dynamic-HASH.js gzip	2.5 kB	2.5 kB	N/A
edge-ssr-HASH.js gzip	258 B	258 B	✓
head-HASH.js gzip	352 B	352 B	✓
hooks-HASH.js gzip	370 B	371 B	N/A
image-HASH.js gzip	4.27 kB	4.27 kB	✓
index-HASH.js gzip	259 B	259 B	✓
link-HASH.js gzip	2.67 kB	2.67 kB	N/A
routerDirect..HASH.js gzip	314 B	312 B	N/A
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	309 B	309 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Overall change	6.63 kB	6.63 kB	✓

Client Build Manifests

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
_buildManifest.js gzip	483 B	485 B	N/A
Overall change	0 B	0 B	✓

Rendered Page Sizes

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
index.html gzip	529 B	529 B	✓
link.html gzip	542 B	542 B	✓
withRouter.html gzip	524 B	524 B	✓
Overall change	1.59 kB	1.59 kB	✓

Edge SSR bundle Size

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
edge-ssr.js gzip	95.5 kB	94.4 kB	N/A
page.js gzip	3.05 kB	3.05 kB	N/A
Overall change	0 B	0 B	✓

Middleware size

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
middleware-b..fest.js gzip	623 B	625 B	N/A
middleware-r..fest.js gzip	155 B	156 B	N/A
middleware.js gzip	25.5 kB	25.5 kB	N/A
edge-runtime..pack.js gzip	839 B	839 B	✓
Overall change	839 B	839 B	✓

Next Runtimes

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
app-page-exp...dev.js gzip	171 kB	171 kB	✓
app-page-exp..prod.js gzip	97.5 kB	97.5 kB	✓
app-page-tur..prod.js gzip	99.2 kB	99.2 kB	✓
app-page-tur..prod.js gzip	93.5 kB	93.5 kB	✓
app-page.run...dev.js gzip	145 kB	145 kB	✓
app-page.run..prod.js gzip	92 kB	92 kB	✓
app-route-ex...dev.js gzip	21.5 kB	21.5 kB	✓
app-route-ex..prod.js gzip	15.2 kB	15.2 kB	✓
app-route-tu..prod.js gzip	15.2 kB	15.2 kB	✓
app-route-tu..prod.js gzip	14.9 kB	14.9 kB	✓
app-route.ru...dev.js gzip	21.1 kB	21.1 kB	✓
app-route.ru..prod.js gzip	14.9 kB	14.9 kB	✓
pages-api-tu..prod.js gzip	9.55 kB	9.55 kB	✓
pages-api.ru...dev.js gzip	9.82 kB	9.82 kB	✓
pages-api.ru..prod.js gzip	9.55 kB	9.55 kB	✓
pages-turbo...prod.js gzip	22.5 kB	21.4 kB	N/A
pages.runtim...dev.js gzip	23.1 kB	22.1 kB	N/A
pages.runtim..prod.js gzip	22.5 kB	21.4 kB	N/A
server.runti..prod.js gzip	51.5 kB	51.5 kB	N/A
Overall change	829 kB	829 kB	✓

build cache Overall increase ⚠️

	vercel/next.js canary	vercel/next.js feedthejim/pages-render-optimisations	Change
0.pack gzip	1.59 MB	1.59 MB	N/A
index.pack gzip	106 kB	107 kB	⚠️ +485 B
Overall change	106 kB	107 kB	⚠️ +485 B

Diff details

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Diff for pages.runtime.dev.js

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Diff for pages.runtime.prod.js

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Diff for server.runtime.prod.js

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Commit: e376f18

[shuding]

Makes sense!

[timneutkens]

This is a bit weird, similarly the this.serverOptions.experimentalTestProxy and this.minimalMode / this.renderOpts.dev are weird too, maybe these should be changed to process.env.NEXT_INTERNAL_X instead, not required for this PR though.

addressed changes