Canonical's Christopher Halse Rogers wrote a blog post over the weekend to try to clear up the XMir performance situation and say that Canonical engineers are working on improving the performance, as users begin to discover there's a performance hit in using XMir.
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."