Hardly a shock. It is intercepting EVERY read, reading an ENTIRE file, comparing it against known checksums (which can take ages to produce a single checksum once from even a small file), and then trying to apply "heuristics" to see if it's doing dodgy stuff - BEFORE it will let you or Windows access any file whatsoever.
Of course it's a resource hog - you only have to look at the path. And the more viruses, the more heuristics, the more opening of files, etc. the greater the time it takes. That's *before* you get into badly-written AV, AV updates that use synchronous DNS lookups, on-the-hour updates and complete disk scans etc.etc.etc.
An AV is there to save you from your own stupidity. If you execute a rogue file, chances are it will DISABLE your AV before your AV even knows that virus exists. I've certainly never seen an AV "stop" a virus in it's tracks on anything but the most perfectly managed setup (and home PC's are nowhere near that category - nor are *most* business setups!).
If you need a program to not only intercept every disk read / write that you do, but to scan every byte of every disk each day, and to update itself hourly, just to stop you RUNNING PROGRAMS YOU SHOULDN'T then you better put up with the performance drag of such a task.
Or you could just learn to keep your *important* software up-to-date (e.g. browser), use secure browsers, not execute things that try to download without your permission, not have a PC that's open to the world (i.e. use a firewall which *doesn't* impact your PC's network access anywhere near as much as you think it might because it only sees IP/Port numbers most of the time and acts on only the initial packet of the connection - mine is an advanced software one and stores a cached list of authorised programs so once a program is authorised, you don't even NOTICE that it's going through a firewall), and not install every piece of junk that ever appears.
16 years. 16 bloody years without a single antivirus program running and the only virus I've ever had was from a very-reputable magazine coverdisk when I was a kid (on a copy of Sin!). Zero damage, immediate detection (by myself), immediate cleanse and removal. Just stop double-clicking on things and using ancient versions of IE to browse the Internet. Follow the rules and no anti-virus is even CLOSE to being practical or useful. That's held from DOS through to my current setup (XP SP2!), none of which had any "explicit" protection that's supposed to save you from rogue programs (unlike Vista, 7, etc. which STILL are targeted by viruses every day!)
In the schools whose IT I've managed, we load the machines with AV because performance isn't an issue and certain regulators like the reassurance but it's still yet to detect a single GENUINE virus (plenty of false-positives) on 150 machines for 450 kids (in my current school) and thousands of desktops / tens of thousands of kids (overall in the last 10 years) before it actually gets shut down - we call it the "canary" effect... when the AV stops calling home to the central server, that probably means it's been transparently and completely disabled by some virus that slipped straight past it. That's about its only real use.
Currently on a 5-year-old XP image at the moment (which has been transferred between 3 actual computers in its life). No AV in it's entire life (but has ZoneAlarm Free edition from the first second to let me go online to get updates, decent browers, etc. safely). Autoplay is off. Never had a virus. Passes all virus scan checks. Show no suspicious activity whatsoever. Worst that happens is I get a dodgy email that *might* be genuine - I have to download it (safe, because my browser isn't stupid), then re-upload it to something like VirusTotal's online scanning service to determine if it's genuine. Happens about once a month or so when someone else's AV goes potty and thinks genuine files are viruses and I have to prove they aren't and / or when someone sends me something that I just don't trust (because they are stupid and probably *do* have a virus).
Stop buying this junk. Stop installing it. Stop supporting this industry that will never "end" while people are making broken operating systems and browsers. Instead, use your brain and don't execute anything you can't verify, and don't use incredibly pathetic programs to go on the net with.