@kiwifruit I've been thinking about your question and think it should be pointed out there are many different factors which I have experienced which have helped me increase my averages and overall performance for want of a better word. For me there is no such thing as one cyclist being better than another, we are all simply cyclists with differing abilities which are influenced by a wide range of factors.
I'm 65 and frankly enjoying the form of my life - a fact not a boast. I enjoy two types of riding, simple bike rides bowling along at +/- 80% of effort and on a flat ride this will average 15/16 solo and around 16-19 with a group and dependent on how we feel on the day. I should say flat here is what some consider hilly. My other pleasure is climbs and hills, sometimes I just enjoy riding them, other times I go for it. For example on yesterday's easy ride of 32 miles, 2500 feet avg 15.1 and included two of my favourite local climbs. Last Sunday I did the club hill climbing challenge and rode 20 miles, 3084 feet, 10.8 avg, knowing the climbs I had to do I ambled along between climbs.
So what are all the different factors which influence our speeds? The first has to be time. I'm four years in to retirement and ride 150/200 miles/week. Every mile helps. My rides vary between 30ish miles for training to 70/80 for a ride and cafe with friends. I've changed both my pedalling and climbing technique. Lost weight. Learned to relax more on the bike. Three years ago I bought a new bike, it makes a huge difference. March 2019 I invested in very good wheels and tubeless tyres, again it made a huge difference, around 2-3mph on the flat. Get your head right, know you can do it. Much good cycling is done in the mind. The last thing is pain! To get better one has to be prepared to ride through the hurting. I don't mean physically damaging oneself but pushing on when your legs are screaming stop, the breathing is hard and all you want to do is stop - next time it will be easier. This aspect is best achieved with a group which runs at above your personal average or comfort zone.
To my mind all of the above influences speed and performance. Different riders will pick out different aspects. I am not suggesting to get better people have to buy better bikes but I am saying in my experience a better bike and good wheels does considerably improve the rider's overall performance.