Wellingborough North will be where the Corby trains normally switch between fast to slow in one direction (don't remember which) so will be used every 30min and justifies a faster layout. Market Harbrough Junction is just the two crossovers, which will only be used in the unusual circumstance of single line working or a train terminating there. Anything using it probably also stops at the adjacent platforms so won't be able to go fast anyway, and using faster and longer crossovers would probably have needed a longer straight section, perhaps tightening the curve beyond and reducing the speed for non-stoppers.
At Derby the sweeping away of what was basically a Victorian layout and the removal of redundant goods lines allowed a major realignment of the south end, and therefore much higher speeds. There wasn't so much of an opportunity at Nottingham, or (back on topic alert!) at Leicester, as bridges and other features constrain how much the track can be re-arranged. I think Sheffield has to await wholesale re-signaling before anything is done (as Derby and Nottingham did) and that in turn will depend on HS2 requirements if Phase 2b goes ahead as planned.
The London Midland seemed noticeably sluggish in seeking out speed restrictions - much of the WCML electrification just hung the wires over steam-age layouts and even later on Crewe and Leicester didn't do much speed-wise. Contrast the Eastern's remodeling of Peterborough. Later on York and Newcastle replaced a huge amount of low-speed ironmongery with a simpler but much faster layout that probably increased capacity as modern trains with better acceleration could clear the junctions more quickly. Unlike Crewe and Leicester, York and Newcastle have mostly stood the test of time and handle today's traffic with only minor tweaks.
