Pick up speed
Definition 1 of 2
Pro Tip 1/3Motion Context
Used when talking about objects or people starting to move faster. 
She picked up speed as she skated down the hill.
Pro Tip 2/3Momentum Buildup
Implies gradual increase in speed, not an immediate change. 
The cyclist gradually picked up speed on the flat terrain.
Pro Tip 3/3Figurative Usage
Can be used metaphorically to describe quickening of events or processes. 
Discussions picked up speed when a solution seemed near.