Watsisname, it's not a frame rate problem! I'll give you the math to prove it!

Distance = 5.01 AU

Time = 30 Minutes

Calculated Speed = 416380 km/s

Calculated Acceleration = 462.3 km/s^2

Inputed into Space Engine:

Acceleration = 556.221 km/s^2 (closest I could get to with the mouse wheel)

Actual speed (from a single press of the "w" key) = 24.xx km/s and this speed remains pretty much constant until further presses of the "w" key are used!

Now can you see my problem? It's not a frame rate problem and yes the math may be correct, however there is something missing to tie it all together!