I finally managed to solve the issue of laggy youtube playback in Safari (with playback in Chrome being perfect). First, I tried uninstalling Flash player, deleting anything Flash related in Library, repairing disk permissions and installing the newest Flash from scratch. This worked usually only temporarily before lagging begun again. The problem as I've come to realize was that I enrolled in Youtube's HTML5 beta, and after switching back to Flash, all of the lag was gone. I don't understand, I thought HTML5 was supposed to be smoother and putting less strain on the computer.. oh well.