Shift up one gear higher than you generally cruise in, and make yourself catch your cadence back up to your usual rpm.

It's very tiring at first, but once you get the bike up to speed it is easier to keep it going there.

When you feel you are getting fatigued, drop back down into your customary gear.

Keep working your way into the higher gears and soon you'll be using them more, and going faster, and passing folks without really feeling the effort.

Gears are magic tricks you can play on your legs!

If you are already at your top gear and getting passed, then you might want to look at chainring options to get you higher gears.