Well, to get a boolean out of a bit array you have to do some operations. So at first it doesn't make it more performant. Compilers probably don't automatically make them bitarrays because of that.

However, the memory savings means less cache used. And a cache miss is way more expensive than those bit operations. So they should be more performant. I'm sure someone out there has done the actual research and there's a good reason why compilers don't make all booleans bitarrays.