Nicholas49,
Delays from first wave are normally due to ATC restrictions. If that specific flight flies through French airspace where ATC are often on strike or "working slow" then you often get a slot outbound and inbound as well back to LGW. If ATC are not on strike you still get the odd slots flying to busy airports.
I have just finished a block of 5 earlies and have except from 1 day had slot restrictions on all my sectors. As an example I flew to Murcia and back (flying through french airspace) and got outbound a 1 hr and 33 mins SLOT- and return another 25 mins, causing delays for the next wave when we're back late when next crew are taking over the aircraft.
Apparently our new CEO wants to have a longer turn around between flights and thereby minimising delays. Some trials are to take place this winter, allocated seating is also rumoured....Time will tell.
Hope that answers your question.