FAO someone from Lenovo - for the Driver dev team.
Download DPC Latency Checker or LatencyMon
Run either utility
Hit the Hardware mute button on the T420.
Watch the DPC Latency Spike to around 4000 micro seconds. Same applies when unmuting. Also occurs when using windows mute option, because it lights up/turns off the hardware mute internal led.
I fail to understand why the hardware driver (simple HID device) needs to queue kernel work like this, it's a simple command that can be implemented using an inverted call pattern to tell other user mode programs (such as the on screen drawing device) that it was successful.
The Windows Driver Development Guidelines state that queued work should take no more than 100 micro seconds.
If someone (ideally a Lenovo driver dev) could pick this up that would be great. Thanks.