.MKV files play very slow on my MacBook Pro (3gb ram). What do u suggest?

Quicktime should be able to play those.

System Preferences/Quicktime/Advanced/Mime Settings - check the box Matroska (a Russian codec with the mkv suffix).

It could be a 'difficult' form of Matroska file. FYI Matroska is just a container like MOV or AVI, and some mkv have the video encoded with Real Video Codec (RV9 RV10). Perian+QT can't decode this, but VLC will play .mkv files just fine on OS X, as long as they don't use H.264!

It's a bit of a lottery until Matroska decide to properly support the Apple platform. I have found this:

http://mkv4mac.free.fr/index.php?lang=en&p=lec

which may help.

But there may be another workaround: 1. Get Info the mkv file
2. Change to extension to wmv. Save as wmv and that could work better.

Could be. I don't have Matroska ticked in Quicktime preferences, as it is also the format used by Russian p-orn sites to distribute malware and trojans.

But it is a fact that not ALL formats are reproducible on a Mac.

JoeyDee wrote:
Ah!

That might be the problem. It does use H.264 which means it's High Deff no? Is that the reason the playback is very choppy?

H.264 does not mean high definition. It can be but you can have very tiny low-def H.264 for e-mailing to people etc.