i know you may think i am going crazy here with this question, but do you have your video drivers for your card installed? or are you still using the generic ones that came with the installation of your distro?
another thing is people try changing the option _snd_mixahead in the config file. it is suggested to use 0.23 as its value as that seems to be the one that helps most people, but i know personally that wasn't the case with me, and although off hand i can't remember the value i used, which to me is probably irrelevant information for you, you'll just have to play with the values, that is if you do have your video drivers installed. if you don't i suggest you download and install them before looking ahead to this option. without your video drivers this lag can occur, as i also personally experienced this.