I believe the main trigger has been when quitting and resuming mid event in championship or career. The ai can very suddenly become slow, 2 or 3 minutes per stage slower on any ai setting.
Personally, I have not had the same issue whilst playing an event all the way through but others have, particularly it seems, when creating an event outside of career or champ with more than 9 stages per event though I haven't tested this. Possible tyre selection issue in this case?
Others have reported random mid event pace inconsistency even when playing an event all the way through but I have not experienced this recently and the only trigger suggested is that it tends to be later in the event when these issues occur..