There is a LARGE difference for traditional hard drives. I went from 40 seconds to 60 boot after installing Windows and the applications I needed on a iMac 27" 2011. In terms of SSDs, it should be a very small margin like 0-3 seconds depending on the size and quantity you have on the windows partition.
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 It uses Windows XP 32bit - because of the outdated IT build (new computers all have Windows 7 Enterprise 64 bit).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.