No slowdowns on my Mac Pro here, it runs fast all the time so it seems.
But at work Bootcamp runs on iMac 27" with Core i7 860 processor and 8gb ram. It uses Windows XP 32bit - because of the outdated IT build (new computers all have Windows 7 Enterprise 64 bit).
I also have Symantec Anti-virus that loads on startup at work in mac OS, and whatever happens on start-up, it is dreadful. Just walk away and do something else, it's pathetic. And then you decide to start VMWare to run the bootcamp configured install of XP. Oh dear. It is appalling.
Starting directly into XP with the Command key at start up is a little better.
So it depends on the machine and the installation/configuration on each machine.