Well much will depend on the power of your system - processor speed/memory etc.
Vista came pre-installed on my Laptop so can't say anything about that.
I have recently upgraded my old (virtually 6 yrs old now) Desktop PC from XP to Vista Home Basic. The PC is a 2.4 Ghz Pentium 4 with only 1.2 Gb Ram so relatively slow by today's standards. I did a clean installation and I recall that it didn't take very long. I've now set the perfomance settings to "Adjust for best performance" and contary to my fears the PC runs faster on Vista than it did with XP. This of course might be because it's a freshly installed OS - time will tell.