x/pkgsite: large module pages are very slow to render

[deltamualpha]

The datadog-api-client-go packagesite page is... enormous. The raw HTML over the wire from the server is over 21MB, and rendering performance is abysmal. It can take up to 25 seconds for the page to become fully responsive to input. Obviously, this is an outlier package, but even given that, there seems like plenty of room here for performance work.

I fired up the Chrome performance tooling to see if there was something obvious:

Nearly all the time is spent in "Recalculate Style", presumably because the browser is constantly re-flowing the page as more and more HTML gets thrown at it?

What is the URL of the page with the issue?

https://pkg.go.dev/github.com/DataDog/datadog-api-client-go@v1.10.0/api/v1/datadog

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/99.0.4844.51 Safari/537.36

Screenshot

No rendering error, so screenshot isn't useful.

What did you do?

Load the page.

What did you expect to see?

Reasonable page-load times

What did you see instead?

An extremely-long rendering block.

[deltamualpha]

One previous issue I could find; #38892. Scale of concern there is completely different, however.

[deltamualpha]

As mentioned in #50911, https://pkg.go.dev/golang.org/x/sys/windows suffers from interaction slowdowns, although that page is a mere 3.5MB.

[jamalc]

/cc @jinhongy @fflewddur @jba

I think we originally limited documentation size to 10MB for this kind of issue but we came to the consensus that slow docs were better than no docs. Even with JavaScript disabled this page is heavy on my machine. Should we consider a text only render method for doc pages to help with cases like this?

[jba]

Is there a way to disable the layout engine until the page fully loads?