From "sprinterisation" in May 1988 through to the mid 1990's Melton, Oakham and Stamford were served every 2 hours off peak... The non stop Leicester/Peterborough services were timetabled for 47 minutes... Since then the linespeed has been very slightly lowered at Manton Junction...
As for 90mph running, there are a few stretches but as soon as you're up to 90mph you need to break for the 75mph stretch... There is also an entirely pointless stretch of SP90 on the down line after Manton Junction (55mph) on a rising gradient followed by a 75 mph on the approach to Oakham... Not a chance of getting much above 75mph, let alone 90mph, before the 75 mph section...