Christopher Halse Rogers basically says that a lot of testing is happening, 10~20% performance drops right now are expected over raw X, and they hope to better the performance. In particular, composition bypass is a major performance win for full-screen applications by not having the unity-system-compositor do extra compositing work, but that code isn't ready yet. There's also some performance hits due to differences in the rendering models between X and Mir. Last but not least, there's some X cursor work to address.
The blog post can be read in full here.
On a semi-related note, Matthew Garret is also out today with a XMir blog post. His latest blog post concludes with, "XMir on Mir in Ubuntu provides no user benefits and isn't a compelling technology demo. Mir itself will permit a range of additional features, but isn't slated to be running a user session itself until 14.10. The only obvious benefit to Canonical in shipping XMir on Mir is to gain additional testing, which makes using it in 14.04, a supposedly stable and long term release, a somewhat surprising choice."