This is a bit of an odd one. My keyboard works just fine (apple bluetooth) with my iMac (27" late 2012, 10.9.1) - typing is rapid and responsive, all the keys work, etc. However, when I press the play/pause key (F8) iTunes ignores it for maybe 2 or 3 minutes and then processes the action as if it were just pressed. During this time the keyboard works fine - I can type just fine, operate the volume with the function keys, change the brightness etc. The system doesn't forget that I pressed the key to control iTunes, but it seems to be slow to respond. This is not manifest by the system slowing down though in any sense that I can tell - even iTunes behaves just fine during this (I can start and stop it with the mouse, change tracks etc). So, for example, I can have iTunes playing, press the key to pause it (nothing happens), then pause it manually by clicking, then have it magically start the track again a minute or so later when the button press is finally registered. I assume this is some issue with iTunes rather than the OS's handing of the keyboard input since that does not appear to be affected at all, especially since all the other function keys are instantly responsive all the time. I have no mods like sound flower or anything like that taking over the keys either. It's very strange.