The one thing that will make a lot of difference in the long run is to not damage your knees by trying to power your bike up an 8% grade at a cadence of 25. It might work now but when you get a couple of decades older you will regret not having treated your body better. If you still want to be cycling then, make some changes now - get a decent gear range on your bike.

Just realize for some of us who do not wear cycling specific clothes doesn't mean they are loose and flapping. Maybe the OP is wearing loose and baggy clothes, but you can certainly get shirts and shorts that are 'fit' and not all flappy. Same thing with shorts, and I still can't imagine how adamantly some are for cycling shorts. Yeah chaffing is horrible, and we'd do anything to prevent that. But a simple $10 or less pair of poly/spandex underwear can be worn under any shorts you want and should just about eliminate chaffing. I wear my bike shorts/bibs too occasionally, but don't see what the big deal is at least at my typical medium hard effort rides, but I'm averaging under 20 mph. Most of my cargo shorts are cotton/blend though, so I do like the bike shorts on hot 80+ degree rides just to manage sweat better. Still looking for a poly version of my cargo shorts I like but haven't found anything close yet.

Summary: It's not about speed, it's about the proper transfer of power from the chainring (sprocket) to the wheel (and multiple sprockets there) for optimum efficiency.
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If that means you go faster, it also probably means the gear ratio allowed you to make the effort. YOU are the motor.