It slows down animations in the game and usually happens to players that have 200hrs or more. Though sometimes it can be less
Heres a better explaination from Wrye's, one of the Wiki's editors, website
presumably Wrye wrote:
"Abomb Animation Slowing" is a slowing a of animations that typically appears around 225 hours into the game (though it may occur earlier or later). Effects are that many animations (spell effects, burning fires, opening/closing doors) have noticeably stop action effects, even while the rest of the scene is rendered normally. In fact the problem is worse in areas with high frame rates (e.g., simple interiors). This is presumably a bug with the game engine, probably something with thread prioritization for animations.
I found out from Jeff when Blackhawk posted a thread about a problem he was getting.