That wheel size sounds about right - checked one of the Topeak computer user guides online...

Are you talking about your average speed?

If you're talking about elapsed time, that usually is really elapsed time and not active time.

On the average speed front - many of the cycling computers out there have the ability to compute average speed based on either active time or elapsed time. That setting is usually something that is in the set up of the computer - not something that you can look at either way. If your computer is set to active time, your average speed will be based on the time that you were actually moving. If it is set to elapsed time, the average speed is based on the time between when you clicked start on the computer to the time you clicked stop - which can include both active time and time that you are stopped.

Of course it is entirely possible there is something wrong with the computer. Is it new? If not, try changing the batteries. If it is... I have had problems with new cycling computers in the past, and the only remedy I know for that is to take it back to the shop where you bought it!
--- Denise