Do you have modern hardware? Is this a fresh install of Win 7 in bootcamp?
If it is a VM i think VMs are simply slow.
I use MSE on all my Win 7 installs (a dozen within the past few months) because I think it is light on resource usage. So, if you are using modern hardware, I suggest a fresh install of the OS, and after OS updates, install MSE. I have had no experience with slowdowns (like with symantec) so if you do, then open resource monitor and tell us exactly which program is being a resource (memory/cpu) hog.