How do I make a mountain bike go faster (commuting)
I started commuting the same way about 8 months ago on a 29'er mountain bike, 2 things made the trip faster and more enjoyable.

Firstly tyres, I changed from knobblies to slicks and it made a BIG difference both in feel and actual speed.

Secondly I got a lot stronger from biking every day.

I eventually got a lighter cheap touring bike which is faster, and better in many ways, but I still prefer the mountain bike for fun.