No. It's not anything keyboard specific.
There's only one keyboard attached (PS/2 wireless). It's just using the Windows drivers (it's a Logitech but I don't have the drivers installed) and to be clear, it only messes up in this one specific circumstance. Otherwise the keyboard works fine, and as soon as you exit Theater View (or do the other things listed in the above list) it resumes working fine. I do also have a RF Remote Control (operated via Girder) but the issue occurs even if the remote is not attached and if Girder is closed.
One thing I didn't try is detaching my USB-UIRT, but that shouldn't really have anything to do with it (and isn't being used for any functions in MC at all). The only reason I mention it is that the USB-UIRT is fairly new, so that could be part of the issue. And, it's certainly odd that I didn't notice the issue until recently, when I rolled all the way back to build 237 and was still seeing the issue. Could it be something weird with the USB-UIRT device? I would think I would have noticed it before I did, but honestly until the recent changes were made to Theater View I wasn't using it on my HTPC a whole lot -- so it could have slipped through. However, rolling back to build 212 made the problem go away, with the same hardware and software setup... So, even if it is the USB-UIRT (which is complete and utter untested speculation), something is happening that is broken with recent builds of MC that isn't with older ones.
Either way, MC is just pegging the CPU core, which is what's causing the slowness (and the process that is pegging it is MC).