The entire video isn't at your computer or player so if you seek or change speed/resolution it takes time to adapt or catch up.
Fix? Play from files, DVDs, BluRays, files or media that doesn't go over your internet connection.
Bob
Hello!
From some time when I watch videos on you tube and speed them up or slow them down they lag.I'm curious why this happens and I really want it fixed.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic