For a PMax campaign launch, unlike shopping we tend to see good serving momentum develop right after launch.
The issue with PMax is optimization time. For the best results, you need to give PMax campaigns four to five weeks to learn, during which time it will be testing performance over different ad channels and adapting. So it takes longer to perform after launch, vs other campaign types. Sometimes the learning period can be as short as 2-3 weeks.
Coming out of the post-launch phase, the campaign has moved onto some form of automated bidding, and is reaching its highest levels of performance since launch.