- Edited
Things I’ve noticed this with aside from stale beans:
Some burrs also do this. If the percentage of fines is low, the shot will accelerate quickly as soon as the puck starts to erode.
Puck prep: distribution of fines through the puck affects how they migrate. A shot will run slower initially if all of the fines are at the bottom of the puck. The basket clogs and the flow accelerates as the fines exit. If the fines are higher in the puck. with some coffees, the puck resistance increases over time. Good distribution and a consistent method is key to dialing in and getting repeatable results with a given grinder and burr set.