Last month, my apartment complex changed its network from a pretty wide open Wi-Fi to one where we must enter a user name and password every 30 days for each machine. Since then, both my MacBooks (2012 MBP running 10.8.3 and 2008 white MB running 10.7.5) have very slow connections. Web pages will fail to load, or load incredibly slowly (as in the progress bar moves an inch then stops). We get bursts of activity (as also seen in Activity Monitor) lasting for seconds (limited to 4mbps down/0.2mbps up by the new ISP). This occurs on both Safari and Firefox. iTunes music store, the Mac App store, Mail, and Spotify all exhibit identical behavior. My iPad, iPhone, and another's Windows netbook connect without any obvious difficulty. We had a rep from the new ISP come to check out our connection, and he believes that the problem is directly related to the MacBooks (a virus on both?).
Things I've tried without change in behavior (with the 2012 MBP):
1. Connecting directly to ethernet (and turning off AirPort).
2. Restarting the computer and the router.
3. Logging in elsewhere in the complex
4. Changing DNS to 220.127.116.11 and 18.104.22.168
5. Deleting AirPort and re-installing in System Prefs
6. Following the excellent "Step by Step to Fix Your Mac" https://discussions.apple.com/docs/DOC-3353 on these forums, including clearing every cache imaginable with Onyx, and uninstalling Java, Silverlight, Flash, and any System Pref plugins.
7. Sorting through Activity Monitor to find anything else running on my CPU of consquence--I don't think so.
8. Confirming good ping and down/up speeds on Speedtest.net with my iPhone
I know little about networking, but I'm learning. Any other suggestions? Why does the MacBook behave differently on this network?