More efficient hardware architecture is only part of the reason. Having low OS overhead is also of benefit. Anyone who thinks all those background apps like AV has no negative impact is out of touch with reality. The PC has to use brute force to get the same performance that a console can do on much less computing power.
Your statement doesn't make any sense... you demonize "AV" yet say "all those background apps", yet both aren't designed and built the same way.
If you read up on thread priorities you'll notice that there's many available for software. Typically processes run at "Normal", and background processes will run as "Background", "Lowest" or some intermediary. This means when the allotted time slice for the AV program comes up it requests resources that aren't currently in use during this cycle, and doesn't utilize resources that are in-use for higher prioritized processes. Anti-Virus programs are all written to be run at the lower process and I've never seen one written to use a higher priority than "Lowest" or "Background". This structure alone debunks any myth or preconception you may have regarding an anti-virus solution resulting in a computer running slower.
Now, something you COULD experience would be resource allocation taking longer than expected. The specific resource that could have such an issue would be RAM allocations. If your computer had high memory utilization OR had inadequate amounts of memory installed then the 20-50MB of memory utilized by the anti-virus program COULD be enough to force your real-time process (read: your game) to cache to the hard drive, rather than buffering in memory. THIS could result in reduced performance of your game, but this has nothing to do with processor overhead, and "low OS overhead" is a myth as well--the thread scheduler used in Windows will be nearly identical to the one used in every OS. There isn't much improvement to be had in this area--you can only optimize so far and the bulk of the optimizations for schedulers occurred almost a decade ago.
As for gaming performance in Windows 7, most people in the thread have it right--there's likely some optimizations made in the performance of driver usage for high-demand components (such as the video card drivers) which could improve things slightly. This is what I'd expect to see over time and has always been seen as newer versions of all operating systems are developed.