I had resigned myself to leaving the mystery unsolved, until your response spurred me to dig a little deeper. Thanks to that spur I've figured out what triggers the problem, which means I know how to avoid it. I bet you can reproduce it by doing what I was doing, as follows: (1) Reboot, then...

Installed TCC LE x64 13.0, and soon found severe system slowdown. Reverted to TCC LE 12.10, and the problem appears to be gone. Now running TCC LE 13.0 (32-bit version), and no slowdown. This may be idiosyncratic. I've had a problem with one other 64-bit application apparently locking up CPU...