Simple copy and paste from wikipedia: Maximum speed: 1,034 kn (1,190 mph, 1,915 km/h) at 40,000 ft (12,000 m)
I reckon you're underestimating the effects of altitude and air density.
Edit: The number you're looking at in the HUD is indicated airspeed or IAS. That's not primarily an indicator of how fast the aircraft is going, but more relevant to how it will manoeuvre in the current air density. Set one of the DDIs to the HSI and you'll see two speeds indicated in the centre, on the left (true airspeed or TAS) *is* how fast you're going. Fly at different altitudes and note the difference between IAS and TAS. The higher you are the faster you're going, even though that's not what it appears to say on the HUD.