I found that terrain has made the biggest difference. My solo average round where I live (Snowdonia) is about 15/16 mph, but having ventured over to the flatlands of East Anglia my average went up to 19 mph.
As mentioned above riding in a group / drafting a larger faster rider can have a big impact.
I don’t think there’s any such thing as a general average speed, but suggest looking more at improving your average over different types of routes and solo / group riding