Fine on New, bad on old
I now have access to Hyrule Warriors Legends on 3DS, set to arrive on March 25 in the west. I already detailed the Wii U transfer process, but I also discovered another tidbit for myself while playing this afternoon that many Japanese players have already mentioned -- it runs like garbage on the old 3DS.
If you have a New 3DS, you're in the clear. The game runs optimally without the 3D effect on with the New unit, at a very stable framerate that rivals the Wii U version. It's fine with the 3D on actually, you'll just get a slight dip -- but it's playable.
Having tested it on my spare launch [old] 3DS however, it's noticeably worse, to the point where I sincerely think Nintendo should have just made this a New 3DS exclusive. It's really, really bad, with sub 30fps performance and no option for 3D. It was so poor, I didn't even want to play through the first stage.
Consider this a PSA, and I'll also mention it in my review next week, which will take the New hardware into account.