I really don’t think there is any system in place. I applied for 3 child renewals within half an hour of each other. I sent all 3 old passports on the same day. 2 were returned within 2 weeks, the other is still waiting to be processed and was sent on the 24th May. They don’t even seem to be processing them by type of application (some new ones on here are done quickly and some not) or by date received. Mine all arrived on the same day.