I asked almost this same question before on the forums and Krisqo kindly responded with this:
Krisqo wrote:Are you trying to playback the files in a media player? If that's the case, most of the time a CPU does not have enough juice to play them back in realtime. If they open fine in Virtual Dub Mod and in your editing program, you got nothing to worry about.
When I open my lagarith encoded files with Media player, it barely plays back the video (a frame here and there), but mostly just the audio. In Vegas they play much better, but I do have plenty of lag. Some of these work for me and I'm hoping they can help you too:
-In the Vegas program, go to Options>Preferences and then select the "Video" tab. You will see "
Dynamic Ram Preview Device max [MB]." Click on it and make it larger.
-If you have the preview window set to "Best" switch it down to "Good" or even "Preview."
-De-fragment your computer.
-Highlighting a small area you just edited and looping it 3-5 times, while annoying, will gradually make a faster preview show up on the preview screen so you can see if you need to change anything.
-Keep rendering each part of your AMV to preview it along the way making sure not to use a lossless codec. Maybe try MPEG. (Of course you'd still use the lagarith for your final render, but for little snippets to preview you don't have to.)
Sometimes I still have an unbelievable amount of lag, but somehow still tolerate it.
Do many others seem to have this problem with lagarith? If so, I'm going to use Huffyuv from now on. People were always suggesting lagarith to me until recently. I'm ordering a new computer next week and I hope things run a lot more smoothly.
Anyway, best of luck, Rapture**! *Crosses her fingers for you*

I really hope you don't have to make clips all over again before you start your AMV. If you can, try those things above and maybe it'll make the speed faster.