^ That.
WMM just gonna freeze. Gotta deal.
It doesn't like sharing power, as mentioned. Also, multiple short clips, large files, and many effects will make it lag more.
I found that clearing out my imported files tends to free things up more. Of course, when I need them again, I have to reimport, but it's a small price to pay.
[edit] Skipped your last sentence, but as far as I know, most video editing programs are going to give you a hard time one way or another.