I've noticed a slowdown, but not in regard to initial clean/compile, that is @same as with 2.9.2.
The slowdown occurs with subsequent clean/compiles where in 2.9.2 caching takes affect, significantly reducing build times, usually by at least a factor of 2.
With 2.10 there appears to be, for some reason, no caching whatsoever. If a project takes 100seconds on initial sbt> clean/compile, it will take the same time for every subsequent clean/compile, not ideal.
Anyway, in most cases incremental builds are reliable enough to avoid full compilation penance, I mean, performance.