I've been playing with the Skyrim 1.6 beta patch and it's recent small update for around 20 hours and I must say the performance has been very good. I'm running it with my existing character who is level 52 (I want to start new when it's official) and all the mods I was currently using and have had few problems.
The game runs better than it ever did, but I have had some CTD's and a freeze. However, I put that down to mods, mainly the removing and adding I've done which is probably screwing up my saves and is my problem, so I'm very optimistic about a fresh game. It's much better than the 1.5 performance and it's constant freezing, possibly due to the navmesh bug as I've been using the same mods without any changes for awhile now, and corrupted saves which so far hasn't happened again. Fingers crossed.
I'm sure, unless it's my imagination, that my character now runs noticeably faster and more importantly, walks faster which was one of my bug bears when escorting or following quest NPC's. At first I thought it was a fps increase and still may be, but I'm sure they increased player walk and run speed as well. If I'm wrong, well I'm not complaining about it. The game in general runs smoother, and load times have reduced noticeably even though they were pretty quick before anyway. I've never had lag anywhere in the game since I first started playing, even at Markarth as some do, so I'm pretty happy at the moment with performance.
I rarely fast travel but when I did with the 1.5 patch I would often get a CTD on arrival or a never ending load screen, and sometimes a few areas would become off limits and I had to find another way round, which I attributed to bad spawns or navmesh issues and those spawns. This hasn't happened so far with 1.6 as I've been fast traveling a bit more just to get this character through the main quest before I start over.
I read the Official forums and the complaints about horse combat and animations, (and other stuff that is mod related), but I wish people would just use some common sense and realize if they opt in to a beta they should expect there may be issues. After all, it's not Bethesda's responsibility to optimize the game around the many mods people may use. The only thing they are responsible for is the vanilla game.