I'm not sure what the precise reasons for this being faster are,
but consistently benchmarking in my project about 50% more type
instantiations with the original version vs. the one proposed in
this commit; plus the compilation time is 20% longer in the original.

The previous commit contained some pseudo-optimizations that had
no realworld impact.

Turns out requiredKeys is not necessary and filtering out
optional keys in the mapped type's key filtering does the
same thing. This also further reduces the amount of type
instantiations which slightly improves performance.