Source video clip works fine but its lagging in AE CS5

Question

I recorded a Maya tutorial using ScreenFlow, the file is uncompressed and 4GB big and it has sound. Its 1920x1200 or something like that. Really big.
When I take it into After Effects CS5 it lags (both in RAM preview and when its rendered), like as if the framerate has been lowered to like 10fps. The sound is working just fine.
I did the exaaaact same thing the day before, but just with a smaller source file (around 2GB) and it didnt lag. The export settings from ScreenFlow was exactly the same. The render settings in AE are exactly the same, its the same composition actually.
I thought it might have been just something with the file being huge, so I went into AE, cut it up, rendered lossless animation but it was still lagging.
I went into Adobe Media Encoder and cut the video up into 3 parts, rendered it lossless and it flows fine, but in AE it still lags, just like before.
Im baffled, I have no idea what to do.
Ideas?
Cheers,
Richard

Answer 1

Ever read this?
http://help.adobe.com/en_US/aftereffects/cs/using/WS3878526689cb91655866c1103906c6dea-7ec9 a.html
Look for the section, "Use RAM preview to play video and audio".
Im starting to believe its just After Effects that cant handle the file (its 4.7somethingGB's, 30FPS, 1920x1200).
File size is irrelevant.  Bit rate is very relevant.  I'll bet you can't play the file properly in Quicktime, either.  Unless you have a RAID on an ESATA connection, your hard drive can't keep up. 
I'd also think that previously to this file, you might have recorded in a different codec, which would explain why you could just hit the space bar and expect miracles.
There's nothing wrong with AE.  You just need to learn how to RAM Preview.