SQL Server 2016 Log shipping restore job stuck
It probably could be for various reasons- 


Compare the volume of data that might would have generated additional log backup size as compared to normal days. Compare the difference between good and bad durations vs amount of data u see.
I am not sure what is the recovery mode- do you allow secondary in read mode by keeping standby ? If yes I have seen issues and slowness based on LS logic to keep switching between no recovery mode and standby mode.
You need to have baseline for other metrics like CPU memory and storage numbers for good vs bad run.