You may find that:
Your video cover images are too big,
You have too many videos (etc.) in one folder or
Your PC's hard disc is struggling to supply all of the information when scrolling.
Mine works fine but I only have about 200 videos in each folder. FYI. I have them grouped in folders such as Kids, Adults, etc. and that's stored on fast 2TB drive. It used to lag a bit on my older 500gig drive though... So it's probably a drive speed issue for you.
Just some thoughts,
I only have 6 folders ie kids movies kids tv doco ect and the biggest folder is the kids movies about 20 files. My new build is running a 1tb sata 3 so i think it can cope. There is just a lag here and there and jumps files sometimes. I use movingpictures for movies and have over 500 and that works fine.??