Are you using thumbnails mode by any chance?
If so then Opus is trying to generate thumbnails for the video files. That can cause a slowdown by itself (although it shouldn't happen if the movie plugin is disabled).
If reinstalling Opus makes things fast again briefly then that sounds a lot like the problem I saw where my anti-virus program was causing Opus's thumbnail caching mechanism to slow down exponentially as the number of cached thumbnails increased. Have a look at this FAQ to see if it helps: