Since Chi migration html rendering is essentially single threaded due to unrolled

[zeripath]

There is a global lock in unrolled.render.HTML:

https://github.com/unrolled/render/blob/b7951a4bb643b89b5e45e179cee661139809c294/render.go#L392

Anything that holds up Rendering just blocks it for everyone and we essentially can only render one html template at a time.

Originally posted by @zeripath in #15826 (comment)

[zeripath]

See: unrolled/render#90

[zeripath]

Fixed by #15845