I've got no real major UI speed issues with Vista, that's because I run it on what is basically state of the art hardware (quad core, 8GB RAM, fast discs, high end GPU, etc.). Although that is definitely out of the mainstream cost / commonality, and WAY beyond the 'minimum system requirements' for the OS, the fact is that it works well for the OS.
IMHO it is WISE to design a FUTURE software product to run optimally on computers that are about 2x to 4x the capability of the highest end units you can commonly find for sale when you start developing, because in 2-4 years those sorts of capabilities will be mainstream in certain markets (e.g. desktop workstation type PCs).
Although Vista runs horribly badly on today's lowest end hardware, and pretty badly even on 2-3 year old mid-range hardware (e.g. single core CPU, 1-2GB RAM, etc.), at least we can be comforted to know that mainstream hardware next year will run it pretty well not because Vista is efficient / lean, but because the average new PC will be twice as fast by then.
They messed up by marketing it FOR use on the low end / midrange systems of 2006 and 2007. They should have just raised the system requirements to dual core CPU, 1.8GHz+, 200GBy+ hard disc, 2GB DDR2 RAM, DX9 GPU better than a 6200, et. al. and it'd have been reasonably decent at meeting expectations.
The PROBLEM I have with Vista even on HIGH END hardware (and it is getting to be a WORSE problem all the time as the hardware gets BETTER), is that I feel like I'm running a NERF OS that just doesn't give me the POWER and TOOLS to do the job of taking ADVANTAGE of my high end hardware. Especially the 1TB + disc drives we have today.
It just doesn't have the SEARCH, INDEXING, METADATA, BACKUP, COPY / SYNC, DISC UPGRADE / TRANSFER, RAID, IMPORT / EXPORT capacities we need for personal terabyte (or even much less!) level file / data management.
I know they've IMPROVED these things since XP, but really it is still a pathetic failure, nowhere NEAR enough seamless easy to use capability to do the job required of it even 5 years ago much less what is here and upcoming.
Windows Home server, Media center, file sharing, et. al. are just bad jokes in terms of data backup / sharing / access / management, etc. This is probably the most critical problem with Vista and Windows 7.
The PC is a complex "thing" you have to babysit, discs get corrupted, files lost, discs crash, you have to reinstall, you lose your programs / data, you can't find anything on your own hard disc, it is near impossible to switch from one PC to another or upgrade / transfer your hard disc effectively.
The PC should be an "appliance" where you can focus on getting the job done and just NEVER have to worry about backup, data organization, program installation, migration,etc.
To get there they'll have to radically change the way everything is done.