[calex59]

Isn’t this in the realm at which WWII fighters began to have structural and aerodynamic failures?

Many WWII fighter aircraft reached speeds in excess of 400 MPH. The problems didn't occur at those speeds but at the speeds obtained during dives, in which the planes approached the speed of sound. The P38 was especially prone to having the controls lock up during high speed dives.

[tanknetter]

The P38 was especially prone to having the controls lock up during high speed dives.

The phenomena was called "compressibility".