the attached parfile does nothing but apply the periodic boundary condition over and over again. The run slows down with time, and spends more and more time in Slab's apply routine (Periodic just uses lots of calls to Slab). Digging further this caused by LoopControl and in particular setting
LoopControl::settle_after_iteration = 0
restores the expected behaviour, namely that the runtime in Pariodic/apply increases linearly with the number of timesteps rather than superlinearly.
I blind guess for the culprit is the std::map inside of loopcontrol. I have done no further digging into the code though.