+1 waste of time.

Learn to spin like a pro instead of mashing like a noob, and you won't have a lot of use for the big ring.

It's way more efficient around 90rpm. One gearing change worth making to most MTBs (that don't get ridden up mountains) is a road cassette; eliminating the huge gaps between gears allows you to stay in your powerband.

But even more important than that is riding position; if the seat is too low and the bars too high you're throwing away speed by the bucketload.

Slick tyres help too. But it'll still be slower than a proper road bike.

If you want to go fast and you're under 90kg, you shouldn't be riding a MTB.