If someone is a slower than average driver, they should realize that Google's estimates target average speeds (since they're aggregating data from lots of drivers), and take that into account when looking at Google's estimates.

I guess it'd be nice if Google over time determined if you're a faster or slower driver than average and adjusted accordingly.

Google's estimates seem most accurate for me if I can consistently break the speed limit by about 15 mph. If I get into a bit of congestion and get stuck at the speed limit then it misses the mark.