To a recreational / amateur cyclist riding more will make you faster. To a pro, already riding as many hours as the day permits, training smarter can help.
Also, slick tires help (that buzz you hear riding down the road at speed is the sound of watts being stolen from you).
Also, a more aerodynamic riding position helps. This is more easily achieved on a road bike than on a flat-bar bike.
The tighter the spacing of gears on your cassette the more efficient cadence range you will be able to find on terrain from flat to up to about 4% grades. The more low-end and high-end gearing you have the more efficient cadence range you will be able to stay within on grades greater than a few percent. It's nice when you have enough gears to satisfy both of those needs.
Shoes with cleats and stiff soles will aid power transfer more than riding along on tennis shoes and platform pedals.
Dedicated cycling clothes will be more aerodynamic than cargo shorts and baggy t-shirt.
But you also have to realize that if you ride among rolling hills or steep hills you will typically have a slower round-trip time than on flat terrain. Because even if you can hit 50 MPH on the descent, the aero drag at 50 assures that you're not making up entirely for the slow ascent you had to endure first. And if you ride on a windy day your round-trip will probably be slower than on a calm day. High or super low temps will impact your speed negatively. Ride distance/duration will have an impact on average speed. Stop-and go rides will have a lower average speed than non-stop open roads. Open road rides will have a higher average speed than neighborhood rides. Rough roads will decrease average speed. High rotational mass wheels/tires will result in lower average speeds in stop-and-go rides than lightweight tires/wheels. Aero wheels will do better in head winds or flat calm-day rides. A lightweight bike will do better on climbs than a heavy bike.
On top of that we sometimes just have off days, or strong days, or days where you're more motivated or less. Sometimes you're better hydrated, or better fed than others. Some days the stars just align.
So average speed isn't a great metric for how you're doing. Power is a better measure, but even then trends over time make more sense to follow than individual rides. The important thing to consider is that it's cheaper to get healthier and more fit, resulting in a modest speed gain, than it is to purchase equipment changes that will get you a marginal speed increase.