Fan blades are basically spinning wings or airfoils.
Depending on their design or how expensive they are, they may rely on pushing air rather than aerodynamic effects at low speeds, and they’re always optimized for a specific rpm.
As it speeds up, the aerodynamic flow takes over, with the rotors creating a pressure differential that pulls air through.
As it gets faster and faster, eventually, that pressure differential reaches the next rotor and the entire thing stops being as effective because now the the second rotor is stalled out. (Only they’re all stalled out because any given rotor is both leading and trailing.)
Said another way, each rotor is passing through the wake of the previous rotor and not pushing more air because the air is already moving with the wing.
that’s a great addon, thank you.
Side note; that’s why they get noisier, too.