Hello @SebastianRowan!

Jobs may occasionally run slower due to factors such as cluster startup delays, data skew or partitioning issues, or overloaded resources. In the Spark UI, you can check for startup lag in the Jobs timeline, review GC time, as high GC can indicate memory pressure.
To spot cluster startup delays, look for a long gap between job submission and the start of any tasks or stages.

Check this out: Spark UI Guide. It provides a detailed overview of what and where to check when diagnosing performance issues.