As others have said, the line is a (relative) rural success - the hourly services from Barnstaple to Exeter Central meet passenger needs and are well-used. It isn't clear why anything needs to change.
IIRC the linespeed is 60mph Exeter - Crediton, 70mph Crediton - Coleford Jct, 55mph Coleford Jct - Umberleigh and 60mph Umberleigh - Barnstaple. Given the frequent stops / slowing for halts between Exeter and Eggesford, I can't see that raising the linespeed is going to save much time.
The features which always strike me as adding time to the journey are the 25mph limit over Salmonpool crossing west of Crediton, and the arrangements at the level crossing and loop at Eggesford. It always seems to take an eon to leave Eggesford, particularly on Barnstaple bound services. If these could be improved, you could probably shave a few minutes off the journey without raising standard line speeds.
There's also a 30mph limit at Umberleigh, which seems to fluctuate between being a halt and a compulsory stop, but as I can't remember a train ever failing to stop there, that isn't really a problem.