Still no real proof if it's locked at 30 or original framerate for N64 games, but from what twitch streams have shown, the frame rate does not drop as much or as hard in situations like explosions not making Jet Force Gemini drop to <15fps, and it's generally smoother than the original games on the N64.
The only original Xbox game, Grabbed by the Ghoulies is not emulated, they just ported the game to the Xbox One with 1080p/60fps.
And the 360 games in the collection, they should work more or less like they did on the 360 itself.