When writing a mystery the pace must keep the reader in suspense and wanting to learn more, but you don't want to keep them out of breath. You want moments that take their breath away.
Pace is not:
Running from incident to incident.
Though together pace and suspense make a good novel even if the plot and characters are not stellar. You can have pace without suspense or suspense without pace but that won't make for a good book
Forward movement of a story at an appropriate rate. It is the highs and lows, the fast scenes, and the scenes that allow the reader to take a breath.
The best way to tell the story.
What will happen next.
Pace builds the momentum and sustains the tension until the climax and satisfying end. Pace is what grabs the reader in the beginning and draws them deeper and deeper into the story. Pace is what keeps the reader reading until the end and sighing as they close the covers.
Pace puts the reader in a trance and won't let them go. It needs to grip the reader, making them care about the principal character and whether or not that character will attain their goal.
To keep the pace flowing and the story moving you also need to make the reader emotionally entangled in your character. Making the reader care about the character is the best way to keep them turning the pages and wanting more.
Pace is the constant forward momentum of the story and keeping your mystery on track. Combine your mystery plot with the subplot meshing them like a well woven rung and you will have the tapestry for a well paced story that will be read to the end and leave the reader waiting for more.