It's not hard to demonstrate that it isn't.

Let's use a calculator at

Bicycle Speed (Velocity) And Power Calculator and plug in some numbers. For simplicity, assume that we have a trip that consists of 1 mile uphill at 4%, 1 mile downhill at 4% and 1 mile flat. Pick "racing bicycle - hands on the tops", plug in bike weight 20 lbs, rider weight 160 lbs, power 145 W (2 W/kg - very conservative for a male rider getting back into shape), leave all else at default values.

The uphill part of the trip takes 7:19 at 8.2 mph. The downhill part takes 2:15 at 26.6 mph. The flat part takes 3:37 at 16.6 mph. Total 13:11 to go 3 miles, or an average speed 13.6 mph.

If I drop the flat part and just keep a mile up at 4% and a mile down at 4%, that still gives me 12.5 mph.

The only way to get down to 10 mph with these premises is to knock the grades up to 8%, in which case our hypothetical 2 W/kg rider will not be asking "why is my average speed so slow?", he'll be asking "why am I so bad at climbing hills?" (or maybe even complaining "I hate hills!"), because 2 W/kg gets him up 8% grade at only 4.7 mph, and even the extremely low gearing of Disc Trucker (26/32 lowest gear) is going to be borderline insufficient.