UPD: Apparently the solution above doesn't always work and it appears to be kinda random, I've found a different thing: just press Alt+F4 and then exit the menu by pressing Esc after the loading's done. Works 100% but takes some time

After experimenting for a bit I've come to the conclusion that long loading is caused by the presence of your personal vehicle. The loading time kinda matches the old wait time that took your mechanic to spawn the car so try returning your vehicle to storage before starting a prep, it should help.