On November 10, GameServers.com increased sv_maxrate to 25000 across all servers to reduce lag. That was followed by a larger update yesterday that improves performance for dual and quad-core processors as well as connectivity with the Black Ops Rcon tool, and adds a "Friends fix for join in progress and Friends tab in server browser."
Treyarch is also looking into "GPU hitching" on certain graphics cards, incomplete server browser results, improvements to Quickmatch joins (results with better ping and reduced lag), and a Zombies mode error that claims the game session is no longer available. Assuming you're running Black Ops through Steam, updates should be applied automatically.
We'll be sure to test the CPU-related patch in our upcoming performance review.