prepare to be ridiculed by a bunch of know-it-all's who will tell you, or point you to a thread where you can read, that paying attention to your average speed is a waste of time, and not what anyone who knows anything about cycling would do.

actually if you do the same route a lot, and if you can track the moving average, then average speed will be useful.

obviously, it's easy to measure, which is precisely what makes it so useful - especially for beginners..

also..., have you ever noticed that in a race..., the guy with the highest average speed wins?

as far as why your average is not improving any more..., i have no idea, but i can tell you i got faster by joining some group rides that were faster than i was riding, and building up to their speed.