>This is what I cannot understand. Can a wireless bluetooth keyboard be really causing latencies like that at the PC level in general?

Dell had a problem with their batteries affecting laptop performance when they began to lose power over time. They were constantly signaling an interrupt to the CPU to service the notification that the battery level was low. It notably impacted performance and the only fix was to get a new battery.

If you are having some device that's constantly interrupting the CPU, it could be slowing it down. It would be visible in Task Manager as some high CPU resource utilization.