Here is a train from today that took 19 minutes between departing Ely and departing Cambridge. It left Ely 9 minutes late, at 11:56 (so three minutes after the Cross Country is timetabled), called at Cambridge North, and then departed Cambridge just one minute late. By the time it reached Hitchin and joined the ECML, it was actually early (only by 15 seconds, but still early).
Yes, I know the relevant CrossCountry at 11:53 was late itself. But had it been on time, a planned 11:56 departure from Ely would still have hit its slot at Digswell. (Assuming that 3 minutes between departures at Ely is sufficient?)
Yes, I know it probably wouldn't work at all in peak commuting hours at Cambridge. But the timetable has many tweaks in it for peak-time.
Yes, I know that in general, even off-peak, this would tight and liable to be delayed. But is a delay at Hitchin, even, say, 50% of the time, better than a planned 9 minute wait at Cambridge 100% of the time? I'd favour the former, but I appreciate that whoever plans this is stuck between a rock and a hard place, and I'm glad it's not me that has to make the call!