Then the name of the game is isolation and elimination. Isolate a symptom, then eliminate variables until you can find the cause. First isolate to either a wired or wireless issue by going wired for a game or two. See how it goes. If its about the same, switch back to wireless, if it completly goes away respond and I will change tactics.
If the symtom occured going from 1.20 to 1.21, then that was probably the UPnP patch that went into place. Try disabling it then fallow the stickey and PS3's support site to open all the ports manually. Give that a couple of games. If that continues, do a Speedtest.net test and pingtest.net test. That way we can see its a problem with general connetivity or just multiplayer.
Questions:
1. any connection between other PC's being slow and a more severe problem with the PS3?
2. Are there any other problems or glitches? Do you play any games on the PC? Any issues there? Youtube?
3. Did everything else stay the same except the change from 1.20 to 1.21? And did you even go from 1.20 to 1.21, or did you start from a earlier version?