It slows down in so many places that it becomes annoying. It happens pretty much every time when there's a lot effects on the screen. Besides that there's, from the top of my head; parts of the roofs of Hyrule Castle near the start of the game, the purple fog section in Faron Woods from start to end, that same spot that was a problem in the originals underneat the giant tree above Lake Hylia, the last part of the first phase of the Stallord-fight, the upper section of Zora's Domain, some parts or angles in the inner courtyards of the Snowpeak Dungeon (and I think Yeto's kitchen as well but don't quote me on that one), randomly in Hyrule Castle Town, some of the outside portions of the City in the Sky and lastly the strangest one, cutting the grass in the upper part of the Zora River immediately after exiting the Fishing Hole takes the game down to a crawl for a couple seconds every time. There's more, but I'm not sure.

LIke I wrote, I really didn't notice any performance issues, maybe little only Faron Woods in same parts, while I notice FPS fowns in WW HD. Do you play on gamepad screen, I remember people saying when you play on gamepad screen game has some fps issues.

Mostly, like 3 out of 4 times. Though I generally play on the Gamepad screen with any game and it's always fine. Would be weird, Gamepad's just a screen after all. Anyway I'm unsure, I'll test that theory.

The only times I noticed slowdown in TPHD was in the foggy section of Faron Woods, and briefly in Zora's Domain when the waterfall covered most of the camera's view. At 1080p, Wii U doesn't cope so well with screen-filling alpha transparencies. For the most part though, the game ran very solidly for me.

Same.  Some of the perceived slow down may be the hitstop that Twilight Princess uses.  When you hit a bunch of guys it can seem like the game framerate lurches.