The number ONE factor and number TWO factor for going slower on your mountain bike are wind resistance and tire resistance. If its an old bike with an inefficient drivetrain then this is important too.

Seat tube angle of a particular number doesn't mean anything. My road bike has an effective seat tube angle of about 69 degrees and I'm faster on this bike than it the seat tube angle was steeper. Seat tube angle needs to relate to your personal fit on the bike.

The OP has given us very little information to go on so there is not much else we can advise him on.

Anthony