Given how often the Brighton/Cambridge services suffer late running and have to have remedial measures such as either missing intermediate stations or being turned short, reducing turnround time at Cambridge really wouldn’t be a good thing.
As things stand it’s quite common for trains to be reversed at Letchworth or Royston, or run non-stop for sections like Stevenage to Cambridge, or sometimes Finsbury Park to Cambridge.
That said, tinkering with things by removing stand time at Finsbury Park won’t help with recovering lateness picked up south of the river.
I use the Brighton-Cambridge trains reasonably frequently. Most of them have a lot of slack in their schedules between the core and joining the ECML down fast line. In the current timetable they are usually booked off St Pancras at xx01/xx31 and Finsbury Park at xx11½/xx41½, and, if on time, usually stand at Finsbury Park for 5 minutes. However, if a Brighton-Cambridge joins the core behind the Sutton-St Albans, instead of in front, most of that 5 minute wait disappears, but the train is still on time departing Finsbury Park.
These days reversal at Royston rarely happens, but reversals at Letchworth are common when there is disruption. At Letchworth the reversal is done in the carriage sidings arrival/departure roads, not in the station, so the running lines are not blocked during the reversal.
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