bad interactive performance when building

[Antti Kantee]

Has anyone noticed that the UI slows down considerably on 5.0 when
building with a high enough -j and doing something which consumes a
little CPU for UI processes (e.g. minimize/maximize a web browser).
This happens both with 4BSD and M2.  I never saw it on 4.99.42.

I realize this may be simply because 5.0 is faster, but I don't know if
we can call it progress if a faster system feels slower to the user.