Here is a quote from a former F-4 Phantom Pilot (John Chesire
What I do know is that at altitude, aircraft are limited by their maximum Mach number , far before they ever reach their maximum indicated airspeed. However, when flying fast at lower altitudes, the converse it true. Indicated airspeed rather than Mach number is the limiting factor. This is because of the thinner air at altitude compared to dense air at sea level.
For an example, the F-4 that I flew was limited to Mach 2.23 at altitude. (1,472 mph, 2,370 km/h). However when flying low at sea level, the maximum speed we could fly in the denser air was only 1.34 Mach (863 mph, 1,389 km/h) because of our limit of a maximum of 750 knots indicated airspeed.
Egressing after a combat strike, we flew “on the deck” as low as we could go and as fast as we could go in our F-4s. Sometimes we even exceeded our F-4's 750 knot speed limit if sustaining heavy enemy opposition... and needed to get the heck out of there, fast! In fact we occasionally came back with the paint on the leading edges of our wings bubbled, singed, and burned by the high-speed and low-altitude air friction heat.