Major DUH moment...I am apparently NOT as slow on the indoor bike trainer as I had thought. I noticed that even though the current speed on my Garmin (Forerunner 305) usually shows in the 14-16mph range at cruising speed, but my average never is more than 13-13.5mph.

I also notice that that average speed will frequently drop to the 2-5mph range for several seconds, even though the cadence remains consistently in the 90-100rpm range on the middle ring.

I did some Google sleuthing and discovered that there is a way to shut-off the GPS sensor for indoor workouts. Apparently my Garmin was grabbing a satellite connection here-and-there and it was canceling-out the cadence/speed/distance sensor on the wheel and messing with my average distance and speed--not in my favor. Ahhh...

Hope this helps someone else. I wish Garmin units shipped with an actual paper manual to reference in a pinch (I'm finding this criticism applies to a LOT of things, nowadays--I'm old skool).