No slow down...in fact...odd as this may seem...when I run Windows 7 on a dedicated drive it runs slower than when within VirtualBox...
Of course I slightly know how to run Slackware and so have it pretty much as minimal as possible...
Ubuntu...great OS...if you like Windows and want something just as easy but for free...but in the long run...Slackware is all one needs...if you don't mind tinkering
All good anti-virus solutions seem to be hogs...I know avast is...but then of course as you open anything or visit any website...you'll notice a decrease in performance as the antivirus is scanning before it completes opening...
One solution I found that works real well is to have three (yes, three) computers...
The first one would be a computer with no harddrives...barebone system with just a cdrom (not a burner, just a reader) for a
Live CD version of Linux (whichever flavor suits you) and a USB port...use this for the internet...no need for antivirus running as there will only be volatile memory to infect...any downloads needed can be done to a jumpdrive...
Second computer is the one you would bring the jumpdrive to...this will only be hooked up to the internet long enough to update the antivirus you have installed on it...otherwise keep it away from a network...use this to scan the jumpdrive...once scanned...bring it to
The third computer...the one you invested the most money...the best hardware...the biggest screen...the biggest harddrives...never EVER hook this one up to a network...do not install any updates (unless there are new features to a program/s you may use)...security updates are useless as you will not hook it up the internet...and no need for antivirus running...perhaps you can initially hook this up to the internet to activate software products but that's about it...the first and second computers don't have to be anything more special than some junker computers...
I DO NOT CONTRIBUTE TO THE DANGERS OF THE 'NET...ignorant or naive users do...don't blame solely the writers of malicious software...its the end-users that propogate it...if you need a license to drive...you should need one to be on the internet...a license to show you know the basics of how computers and the internet work so that we have less people clicking on what should be obvious dangerous items....if everyone on the net knew better than to click on the popup that says "You have been infected" because obviously they didn't have the time to scan the computer to know in the first place...less people would become infected...