I just noticed... and it does not happen consistently on my system... But with SS open as a window and SS clock turned on..moving the mouse over the clock in the task bar will bring SS to a crawl. Sometimes.
I can not reliably duplicate this but when it happens the frame rate drops instantly to ~4.. moving the mouse out of the range immediately returns the rate to what it was. Keeping the mouse over the clock and in some brief time (20 seconds?) the frame rate increases to normal again..