One thing I've noticed when reproducing this is that the PWA load causes Gecko to launch a second content process even though the first one was sitting there unused.
Bug 1641715 Comment 12 Edit History
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One thing I've noticed when reproducing this is that the PWA load causes Gecko to launch a second content process even though the first one was sitting there unused. We also ended up with both content processes set to `FOREGROUND` priority, which should not happen.