Since, as of right now, the attributes and tag type of a jsx element do not play into the expression type, this defers checking of jsx elements. This deferral allows us to know that it is safe to cache many intermediate results while checking those tags (as we will not be in an inferential or contextual context), which we now do. We cache the result of getJsxNamespaceAt, if possible (as that resolution took a measurable slice of time when done as often as it was, since we were doing it a few times on the same tag name), and the sourceAttributesType (where we were spending slightly more than half our jsx checking time when it was uncached, as calculating the attributes type was done both during overload resolution (for stateless components) and checking). In extreme cases like a project that was shown to us, I see an order of magnitude improvement in semantic diagnostic time - in one file down from 4s to 0.5s.

This probably fixes #25085, since it seems to fix a similar issue in another project, though I can't know for sure.

For future work in this area, resolveCustomJsxElementAttributesType is about the only jsx-specific thing left that takes a measurable slice of time (though not very much compared to what I just started caching). It already caches the signatures it uses internally; but it may be worth adding a second layer of caching to it in the future.