yea your right about avchd taking more power but that might not be his issue. He may need a faster computer but vegas is also having problems decompressing the .mp4 container. My computer runs the highest avalible bit rate(24mb) of .mts like butter, however when i try to work with my go pro footy, it crash's vegas every two seconds. hopefully they will come out with a fix soon, until then i recommend proxy editing or just recompressing and saying fuck it to the loss of quality.
at least vegas will open it, premeire thinks that its just an audio file.