I've been trying to find a solution to this since Summer, when I bought SL. First of all, my specs: OS X 10.6.2 2.16 GHz Intel Core 2 Duo 3 GB 667 MHz DDR2 SDRAM (3rd party) Macbook 2,1 (white) 13" I did a clean install of SL. Whenever I am trying to play 720p or 1080p .mkv files in the following player: VLC (all the recent versions, also older ones, in 32bit and 64bit) Plex MPlayer Mplayer Extended Quicktime X with Perian Quicktime 7 (Pro) Chroma Movist NicePlayer playback is lagging like crazy. Sound is ok (but out of sync, obv.), but it's unwatchable due to the lags and artifacts. I have gone through mac-forums basic troubleshooting and I tried all of the tricks suggested on the VLC forums. Most recent thing I tried: converting to .mp4 with QT 7 Pro and HandBrake, but the conversion would have taken 1 hour, so that's not a solution. Oh, and the problem is even worse when playing it on the external 24" screen. Since I know that I'm far from being the only one, has anyone found a solution to this yet? Apart from going back to Leopard, of course.