dig -u is extremely inaccurate, especially on machines with the kernel timer tick set at 100Hz
The current isc_time_now uses CLOCK_REALTIME_COARSE which only updates on a timer tick. This clock is generally fine for millisecond accuracy, but on servers with 100hz clocks, this clock is nowhere near accurate enough for microsecond accuracy.
This makes the dig -u command report very inaccurate timings. On a fast network with sub-ms responses, the reported time will oscillate between 0us and 10000us. I have created a patch to make this use CLOCK_REALTIME if the -u option is passed to dig.