The Qt docs say that creating a timer with an interval of 0 creates a special-case timer which will only fire when the app enters idle - ie when there are no more events in the queue.
When we do this we find that the timer fires continually, eating up CPU time. We are careful not to hang any heavy processes off the timer callback, but even if we don't do any work at all in the callback, we still see CPU usage going through the roof.