>It's good for bitrate starved files, but x264 is actually still more efficient for transparent encodes. x265 currently can't handle noise very well.

Yes this is my impression as well from the tests I've made.

>x265 is also much slower than x264, but it's getting faster all the time.

It will remain slower than x264 though, it uses more cpu consuming algorithms in order to cram out more 'quality per bit', that said it likely has a lot of performance improvements left to do.

By noise, do you mean something like a grain filter? If so, that blows up the size of x264 encodes as well -- enough that people will usually go out of their way to remove said grain to improve the encode.