Bathtubjp_Do you have an OpenCL capable device? Try enabling gpu acceleration (sony).
Is it really a video or image sequence? Is it just in an avi container as uncompressed RGB? Decoding speed for uncompressed RGB is slower than using something like lagarith/UT Video.
I'll try the first one and see how it works out. It is a video, and I compressed it using virtual dub with the codec x264/H.264 (I don't know the difference between these two terms)
Are you limited in disk space? I wouldn't use x264 (especially it's vfw variant) as an intermediate format. Encode to Lagarith. Or don't encode at all and just import the image sequence.
x264 is cpu intensive to decode, it's probably exacerbated with an NLE. Though I think your resolution shouldn't cause decoding issues.
I read from google searches that sony vegas cannot work with x264 in an AVI container. It's recommended to use Virtualdub 1.10.x's external encoder (x264+NeroAcc>mp4box) or Handbreak to pack the video into compatible mpeg-4 container.