So far the game has been running smoothly in terms of FPS on my machine (W10, i5-2400, 8gb ram, GTX950 2gb). The 
two-screens loading problem that I had before is gone with 64bit.
But there are many immersion breaking bugs so far:
- Missing textures in many places/pets/horses/characters. Glowing purple colour is shown instead.
- Missing parts, legs, helmets, hole in the head etc.
- Extremely slow loading of high-res textures. It takes about 0,5-1sec for a hi-res texture to load when you approach NPCs or players (apparently this is not 64bit-only related problem but still, hi-res textures load faster in 32bit)
- Music can totally stop in the middle of a game.
- White noise can be heard out of nowhere.
- Weird and inappropriate echo/reverb on some NPC voices.
Suggestion for the future: Make an option inside the launcher to choose 32 or 64 version.
Sadly, the game is unplayable in 64bit in the current form, at least for me.