[sneak]

The "brain" to which it refers (on x64 machines) basically is an iOS device (running a variant called bridgeOS) on an ARM coprocessor (T2) inside your computer. Same secure boot, same TSS signatures, etc.

It has to update two OSes now, not just one.

[radicaldreamer]

Even the new studio display is an iOS device and also takes forever to update.

[icedchai]

I recently got a Studio Display and its updates seem relatively fast (at least compared to the Mac itself!)