I have a totally modern system, that runs games like World of Warcraft, Oblivion, etc. etc., like butter.
It's not the newest, hottest PC you could possibly buy today, but it should be MORE than ample in every regard for UO.
Yet, in the KR client (which of course I had to use when I first returned on the free trial download, which is the KR client), I have .... "a little lag, all over." From pushing a button to opening a bag. And if I'm near CD, or really any heavy combat (say a champ spawn) this bizarre thing happens where the game will choke, just for a moment, and then my character is trotting off in some direction--- even if I was standing stationary before. And it's hard to get him to stop. I think I SORTA figured out the macro system, but, no matter how I set them up, they bug up and stop working after so many repetitions. I end up having to clear them and reset them from scratch to get them to work again. I have no idea why that happens.
To me, the only thing BETTER about KR is the minimap (shows healers and such, and your body) and SOME elements of the crafting menu. In overall UI and game performance, I consider it terrible especially considering it was supposed to be an "upgrade."
I don't have a '97 computer using Win95, and I still have many issues with KR in terms of performance and bugs. And frankly didn't care enough for the superficial facelift of the graphics KR has to trade it off for functional performance and consistently working macros. So off it went. Mondain's Legacy and I never looked back.
As long as 2d is supported, I will remain in 2d. Even if that means there are areas I cannot access. If 3d is "required", I may duck back out. I don't like WOW as much (anywhere near as much, actually) as UO. But if I'm going to be forced to play a pseudo 3d poor interface/UI laggy UO client, I might as well be playing a 3d good interface smooth running WOW client for the same money.
P.S. There are other bizarre mechanics differences between the games that are incomprehensible. Like why can't I figure out how to undress an NPC vendor in the 3d client? Why doesn't dragging clothes off the paperdoll work like in the 2d? I still have no answer for this and guildies have had me redress their vendors for them if they use KR.