I get slowdowns up to 57 fps on Charlie's stage and also with a few lv. 3 Ultras (just did a lv 3 Shoryureppa)
In wich situations you get the slowdowns? I take it you're using the cia version, aren't you?
I'm using the 3dsx, I get slowdowns pretty much in every battle, although those slowdowns aren't too often.