When Mumble loses the DirectSound device for both input and output, it causes lag in the system including slow responsiveness and jumpy mouse. In my case I changed multi-streaming settings on my Realtek HD for either input or output. There is no indication of unusual CPU usage from mumble.exe. When going into the config and resetting the device, the lag goes away. Nothing was logged in Console.txt at the time. I should note that I run Folding@Home at 45% per CPU core (idle priority), which can cause a bottleneck in certain circumstances; Mumble runs on both cores at high priority. Closing the Folding@Home processes eliminated most of the lag. I would guess Mumble is making short but high demands on the CPU, too short to be easily seen in Task Manager but sudden enough that Folding@Home can't get out of the way quick enough to prevent lag. Since many games or programs use a similar amount of CPU as Folding@Home, I wouldn't be surprised if the same problem would occur in other likely situations.
Mumble version: 1.1.8 and snapshot 2009-04-29-1900-cff157
OS: Windows XP Home SP3
Audio input/output: Realtek HD