No, Parallels is visualisation software so you will get near native speeds, Boot Camp natively boots Windows so you will get the same speed as you would on a PC. With Parallels you also don't the the 3D hardware acceleration but they are working on that for a new release. Now, I've never used Parallels or Boot Camp because I own PPC Macs but I don't think you would notice that much of a difference between the two unless you are gaming, perhaps someone else can vouch for that.
I have a 15" MacBook Pro, 2.0 GHz, 2 GB RAM, 7200 RPM hard drive, Radeon X1600 graphics with 256MB of video RAM. I also have a PC laptop, 2.26 GHz Pentium M, 1 GB RAM, 7200 RPM hard drive, integrated Intel 845 video.
Through Parallels, with 1 GB of RAM assigned, my 'single-core' 2.0 GHz processor benchmarks faster in every category than my native PC. I can view HD WMV and H.264 videos at higher frame rates in Parallels than on my PC notebook running natively. And my PC has a 266 MHz faster processor.
Obviously, Boot Camp will be faster, since you get direct access to the X1600, all of your RAM, and both cores. Now I'm curious as to the purpose of this thread. Surely you knew Boot Camp was faster, and you were going to reply 'I'll stick with Boot Camp, then.', so why did you bother starting this thread?
Boot Camp and Parallels really have two completely separate target markets. Boot Camp is for people who want to run Windows, and only Windows, on their Mac. Parallels is for people who want to access Windows occasionally, without rebooting.
I have both on mine. Some tasks (like Windows gaming,) I run Boot Camp. But not often. For most 'I need to boot Windows real quick to test something' purposes, I use Parallels.