Exactly how much slower is the PSP version? Is it occasional lag, or is everything in general a little more floaty? But I guess more importantly, is it detrimental?

It's a little bit slower than the PS2 version, but fast enough to satisfy you. For example, Judas' Shouha Rekkousen and Reala's and Harold's summonings will be noticebly slower. But everything else is pretty quick enough. I've never played it though, just sharing what I heard from actual people who played it.

Shouha Rekkousen isn't really slowed down, just Harold and Reala's spell Hi-Ougis are slow. But everyone has new Hi-Ougi cuts ins now. even...

*Spofortunaers*