According to Kotaku, the PC edition of Call of Duty: Black Ops is suffering from a massive bout of lag.
The report says that in some moments of the game, it would “slow to a crawl in relatively quiet areas of the game, while in hectic stages full of explosions it may carry on just fine.”
This also applies to the single-player cut-scenes as well as multiplayer.
The issue may be down to just a memory leak in the CPU.
There’s a video of the lag in action below.