I have the same exact problem. The menu becomes laggy if you choose Steam tab like Library but if you choose Community tab the menu becomes responsive. I'm using Nvidia 319.17 with Composite disabled (had the same issue with older drivers).
Tray icon menu slowdown also happens when Steam window is minimized, even in Community tab.
Same thing here with gnome 2 / MATE on ubuntu 12.04. It seems as if the tray menu only updates once per second (while other tray programs right next to the steam icon update normally).
Video here: http://ogc.mine.nu/steamtray.mp4
Nothing extracted yet.
I'm using Xfce 4.10 with xfce4-panel and the Notification Area plugin. When I right-click on the Steam icon, sometimes it takes a while to open the menu and the highlighted mouse-over selection in the menu fails to move along with my mouse.
Edit:
I'm using minimal built-in Xfce compositing with Nvidia 319.17 drivers.
Edit: thanks to healthycs16:
It only becomes responsive when the Steam window is OPEN and on the community tab.