Vita user here. I've got about 4 hours into it. For the most part, I haven't seen much slowdown. Maybe a little bit here and there when enemies really flood the screen, but otherwise its fine.
Controls are ace. My amazon is DESTROYING enemies with sick combos. Vita's God-Tier buttons and sticks really let you do everything you want without any "oh crap I pushed the wrong direction" stuff.
Graphics are incredible. I know it looks great on a TV, but the Vita screen really pops.
Haven't noticed the difficulty so far, but to be fair I've replayed a few dungeons 2 or 3 times for side-quests, so I think that has helped. I'm currently lvl 14.
All around, I really dig it. My only concern is that I've heard there are 9 stages and I've already been through like 5 of them. I realize there are 6 classes, but I could see 9 stages getting old over time. Hopefully the enemies scale? I don't know.