Have you tracked your latency while playing? Large numbers of units needing to load leads to a spike in data, and if your latency is to high the game will begin to lag as it waits for all that data it needs to get there. This data would include things such as NPC/PC movement, abilities used, etc. For TF2 I generally play with a latecy of 20-30ms, with anything below 150ms being the standard I have always heard for accurate/smooth gameplay. I have not played GW or most MMOs in years, but until a few months ago I was an avid WoW player and any latency over ~100 caused distortion that I could notice in busy areas or high-paced fights.
The solution to this would be tracking down network problems. Update network drivers, identify/adjust your max MTU, make sure they have firewall clearance and possibly forwarding from the router, etc. I'm not quite sure what the best means of this would be for Windows 7, but just watching your latency would be where you want to start. Most games have internal latency(ping) trackers. For TF2 go to the console and try "net_graph 3" if I remember correctly, or simply look at the score sheet.
edit > Btw, there have been many complaints that I have seen about the internet being slow in Windows 7, especially in the torrent community.