Otis to the WTC
  The first base to finally scramble interceptors was Otis in Falmouth, Massachusetts, at 8:52, about a half-hour after Flight 11 was taken over. This was already eight minutes after Flight 11 hit the 
North Tower, and just 9 minutes before 
Flight 175 hit the  
South Tower. 
  According to NORAD, at the time of the South Tower Impact the two F-15s from Otis were still 71 miles away. Otis is 153 miles east-northeast of the WTC. That means the F-15s were flying at: 
    (153 miles - 71 miles)/(9:03 - 8:52) = 447 mph     That is around 23.8% of their top speed of 1875 mph.
At 9:11 the F-15s finally reached the World Trade Center. Their average speed for the trip was:     153/(9:11 - 8:52) = 483 mph     That is around 25.8% of their top speed.   
Langley to the Pentagon
  The F-16s from Langley reached the Pentagon at 9:49. It took them 19 minutes to reach Washington D.C. from Langley AFB, which is about 130 miles to the south. That means the F-16s were flying at: 
    130 miles/(9:49 - 9:30) = 410.5 mph     That is around 27.4% of their top speed of 1500 mph.