There is big difference in journey time.
If the do a bit of geometry on this you get two triangles -a high level one and low level one but the angles are the same in both. ( Easier to draw it but doing that on PP is beyond my abilities)
The distance line in the 50,000 feet case will be much much much longer than the lower level line.
However the difference in speed is at best only about 2 or 2.5 times for high versus low and this cannot compensate for the much greater distance flown.
indeed this iswhy since WW2 military aircraft have flown at low level so the transit time and reaction time from Triple A or missiles is extremely small compared to being able to track and aim carefully at something much higher up even if it is going a lot faster
PB