That's exactly how mixed service out from central London is supposed to fit together. In this case the TL trains (fasts) pick up passengers from intermediates like West Hampstead and St Albans travelling beyond Bedford. They can not take the slows until, after St Albans - which in practice means Harpenden South junction, so if the service is running normally, the Corby will have nearly caught up by the time it makes its stop at the Airport. Similarly, the earlier TL fast is arriving at Bedford ready to deliver its passengers picked up from Flitwick etc., who can board the Corby there. The same happens with the Corby picking up from Wellingborough for changing at Kettering and onwards northbound, which is eased slightly by the use of the slow platforms at KET, but that increases the minimum connecting ti,e for passengers, so it is a compromise.
Any delay can cause this to breakdown including late up EMR trains delaying down TL services from taking the fasts at Carlton Road Junction crossover. Overall, the MML working timetable is a work of art as much of the route is operated as sections of two track railway, yet the number of significant delays is relatively small. Equally, there is very little chance of speeding up any of the three types of train that use the lines without making the operation far less reliable.
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