I've just finished an 8 minutes project on After Effects CS5, but for some reason, every time i render the file, it seems to have a bit of a lag when the video is being played. The footage was taken on a Canon T2i. Does anyone know what may be the cause of this problem?
You need to read up on rendering and output. I'd guess that you're using the default Lossless settings. Lossless files are not intended for playback, they are intended to maintain quality through the production process.
Without knowing your current render settings, this is going nowhere, don't you think? Could be anything from unsuitable compression settings to genuine audio delay introduced by AE, but you realyl need to tell us what exactly you are doing...