Running an operating system within an operating system does use extra resources. You must allocate system and video memory.
The main thing I have seen causing slow performance is incorrect default VM settings. I dread installing the guest operating system to have it run sluggish in the VM. Many of the default settings are incorrect for both VMWare and VirtualBox depending on which OS you install. I also had problems with mouse integration in VirtualBox which has worked itself out. I'm not certain if I fixed it or if it fixed itself.
For example, I installed Linux Mint 14 in VirtualBox and with one setting change it ran great. I installed Ubuntu 12.04 and spent a couple hours trying to get it to run correctly.
Once the settings are correct, it runs great on my computer. I could maybe run Windows 7 and two guest operating systems at once and then I would probably be out of resources. Different variables would quickly change this though.
It's a pain in the rear to set up. Once the settings are correct things run great.
Before trying any of this, you need a backup plan other than restore. In fact, I have restore turned off and am using Seagate for daily backup of personal files/folders and Windows 7 system image for weekly system b/u.
EDIT: I want to add, before running anything in a virtual machine, know your license agreement for any software run inside the virtual machine as well as your resident OS.