FileVault + iMovie = Trouble

I was trying to import some video I shot at the beach into iMovie and it just wouldn’t go; the picture would lurch and stutter and then the import would just halt, no diagnostics. I was puzzled because I used to import all the time on my much-slower previous PowerBook. Then I remembered that I’d turned on FileVault, i.e. everything in my home directory is encrypted. I wouldn’t be surprised if soaking up data at FireWire rates, plus encrypting it, might be a bit much; so I logged into another—unencrypted—account on this computer and it imported fine. So I may be jumping to conclusions, but this looks like a bad combination.