Was cursing how slow the UI was on LinuxMCE (grumble... what did they do to the YouTube video to make it look good... speed it up?).
Then I noticed that the red "receiving IR" LED was on all the time on the MCE USB receiver.
Moved it into a dark spot away from light from the fluorescents and everything suddenly sped up. I am talking response times going from 10 seconds from button push on the remote until the UI registered it, down to instant.
I am thinking that the receiver was getting a lot of noise and causing a huge amount of traffic into the USB port.
Also, have had plasma TVs do the same sort of interference (though it did not cause the same slowdown as it was to an IR repeater unit).