It's been looked at by (at least) the GNR, BR, Network Rail (and possibly Railtrack as well) - it's always ended up in the too difficult/expensive box.
The GNR developed the Hertford Loop line 100 years ago because it was better value-for-money (in terms of providing more capacity) than widening the viaduct and tunnels.
It might well be cheaper to straighten out the loop for higher speeds than do the widening.
I quite agree, I like to see work done to raise line speeds on the entire Hertford Loop line and not just because I have to use it...
Down Hertford for Passenger Trains
Alexandra Palace to Bowes Park 30mph
Bowes Park to Enfield Chase 70mph
Enfield Chase to Langley Jct 75mph
Langley South Jct to Langley Junction 60mph
Langley Junction to Stevenage 40mph
Down Hertford for Freight Trains
Alexandra Palace to Bowes Park 30mph
Bowes Park to Winchmore Hill 50mph
Winchmore Hill to Grange Park 40mph
Grange Park to Langley Jct 60mph
Langley Junction to Stevenage 40mph
Up Hertford for Passenger Trains
Stevenage to Langley Jct 40mph (Down Hertford) 75mph (Up Hertford)
Langley Junction to Langley South Jct 50mph
Langley South Jct to Winchmore Hill 75mph
Winchmore Hill to Bowes Park 70mph
Bowes Park to Alexandra Palace 30mph
Up Hertford for Freight Trains
Stevenage to Langley Jct 75mph
Langley Junction to Langley South Jct 35mph
Langley South Jct to Gordon Hill 60mph
Gordon Hill to Bowes Park 40mph
Bowes Park to Alexandra Palace 30mph
There were accurate prior to the conversion of the Down Hertford between Hertford and Langley Jct for ERTMS testing purposes but you can clearly see why the Hertford Loop is neglected line speed wise aka why it takes longer to travel across it however I'm sure if it was possible to raise line speeds surely it would have been done by now or at the very least looked at as a value for money alternative to working on Digswell Viaduct.
Least if faster line speeds were brought in then maybe it might be attractive to run a semi fast service that way as a third semi fast using Hertford and Gordon Hill instead of Knebworth to Hatfield/Potters Bar
