My experience is that you're trying to allocate too much memory to the virtual machine. With 8GB of RAM, I am assuming you're running Windows 7x64, you shouldn't allocate more then 5 or 6GB to the VM, otherwise you start starving out OSX and it's applications which will really reduce performance. It is better to run out of RAM in the virtual machine. If you're running 32 bit windows, don't bother allocating more than 3.5GB because Windows can't use more than that.
Yesterday I had to run two copies of Windows 7x64 (at the same time), both with 4GB of RAM allocated to it and it dragged my system WAY down. But I knew I was paging like crazy because I only have 8 GB of RAM in the system.
And to answer your question, no Parallels won't help that much if you do the same thing. When you set up the VM you have to make sure you don't over commit your system, otherwise it will perform poorly. If you have 2 Cores in your machine, only give 1 to the VM. And don't give more then 75% of AVAILABLE RAM to it either (not just what's installed in your machine). If you're using 4GB in OSX and you have 8GB in your system. You can't give 4GB to the VM because it allocates it all at once and you'll starve out OSX causing lots of issues.
Good Luck.
