I remember reading that 9Fs could reach quite a speed if they were standing in for a failure and when pushed. I'm sure I read somewhere that speeds of 90 mph had been known on occasion.
The story went something like this: Some time in the latter part of the 50s, a respected train recorder (a la Ian Allan mags) was on an up ECML express which had an ailing train loco. (I forget whether it was steam or diesel.)
The loco was taken off at, I think, Doncaster (which had an allocation of 9Fs) and the train restarted, picking up time from thereon with some 90 mph running. The logger had not left the train at Donny, and, from the back, assumed the loco was an A1 or other regular pacific.
It was only on arrival at KX and walking past that he realised it was a 9F on the front.
The WR also used Evening Star on passenger duties on the main line, i think it hauled The Red Dragon on occasion. AFAIK, that was not a sharply timed express (despite being billed as the main up 'business express') - nonetheless, it would have required 70-75 mph running i should think. The LMR also used them on summer reliefs/holiday specials.
Some combination of the ECML run and the WR usage supposedly stirred the BRB to issue an order to ban 9Fs on passenger duties, or at least to put a restriction (70 mph?) on their use. Perhaps someone else can confirm the details.
The WR/SR also used 9Fs on the S&D, of course, but I doubt they could get above 55 mph on that road.