Background threads slow down Fenix startup because we have many more active threads than CPU cores during those first two seconds of startup. So the actual startup-blocking work is getting descheduled because background threads are stealing CPU core time slices.
Bug 2003950 Comment 1 Edit History
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Active background threads slow down Fenix startup because we have many more active threads than CPU cores during those first two seconds of startup. So the actual startup-blocking work is getting descheduled because background threads are stealing CPU core time slices.