Yeah honestly splitting most of the `configure` checks into multiple threads is definitely possible.
Caching between projects is even a straightforward extension with systems like `Nix`.
The "gotcha" here in both cases is that existing scripts that are living in source tarballs are not feasible to "regenerate" in the general case. You could have this ship out with future projects though if project authors updated to new versions of Autoconf.
If you have a particularly slow package, you can optimize it in a few hours. Largely this means "identify which tests 100% match the standard implementation of a check" in which case you can fill in a cached value. But what I think y'all are asking about is "can I safely use a cache from one project in another project?" and the answer there is "no not really - and please don't because it will be a nightmare to debug".
The nasty part about trying to naively share caches is that it will probably work fine ~90% of the time. The problem is that the 10% that misbehave are high risk for undefined behavior. My concern is the 0.5% that appear to work fine, but "whoops we didn't know project X extended a macro without changing the name - and now an ABI conflict in `gpgp` appears on the third Sunday of every October causing it skip encryption silently" or some absurd edge case.
I think optimizating "freshly generated" scripts is totally doable though.