Slow GWR trains near Southall

Apologies if this has been posted before but couldn't find it anywhere

I travel between Bath and London on a weekly basis, and have noticed that , when heading out of Paddington, the GWR trains always slow down between Hanwell/West Ealing , and go slowly until passing Hayes and Harlington , before speeding back up

This has led to delays of up to 10 minutes on most of the trains I use, and did not previously use to occur

Is there any reason for this ? , It seemed to have started post December

It is normally because the 125mph capable GWR train has caught up with the 100mph Heathrow Express train with a 70mph junction to negotiate at Airport Jn, so things bunch up a bit there. Or the train has caught up with another train that’s caught up the Heathrow Express. Or the train has caught up with a train that’s caught up with a train that’s caught up with the Heathrow Express, and repeat.

I expect Mintona has it spot on. Do have specific times and dates? PM me and I can look into it.