Is it a MacOS normal behavior to switch between desktops slower while in Mission Control mode?
This is more of a performance issue than anything else.

Bringing up Mission Control is already a pretty complex split-second animation for your Mac to handle, especially if you have several windows open in your current desktop.

And if you have multiple desktops with several windows open, it has to render all the windows, grouped by application per desktop, and give them a different effect than usual (drop shadows are applied on the window group rather than individual windows in Mission Control) and so on.

Basically, it's a more complex task for your Mac to do, and since you're on Mojave, system UI is largely Metal, so if you have an nVidia chip or older intel graphics with no Metal 2 support, unfortunately it will feel like molasses.

The only solution would be to upgrade your hardware, or turn off the special effects by going to System Preferences -> Accessibility -> Display then checking reduce motion, reduce transparency, either one or both, depending on what you are willing to give up and how much of an improvement checking the options yield.