I'm going to answer my own question, because this will, almost certainly, come up for others.

The problem is with Webroot's SpySweeper, an otherwise excellent malware protection program that is supposed to be Vista compatible. If you leave the "Common Ad Sites" shield on, FT will take forever to load. It may be because of a Javascript issue and it may be because of the number of ads on FT -- I'm not sure. However, turning off that shield restores zippy webpage loading in Vista.