I recently changed my Schwinn road bike from 42/16 (70.9 gear inches) to 40/16 (67.5 gear inches) and I think if anything I’m riding faster and stronger overall at the lower gear.
I made this change to help reduce long term strain on my knees, but it seems to also be just as effective. Also, our trails are a lot rougher after massive flood damage and repairs, so average speeds tend to be down a bit and there is a benefit to slightly lower gears in negotiating those routes.
Thinking about also changing the RockHopper from 42/16 (68.3 gear inches) to 42/17 (64.2 gear inches) and see how that goes.
Otto