As Giri states perfectly already, you can use the code example given in this previous forum page that I have used myself and it works fine for both desktop and mobile.
https://www.wix.com/corvid/forum/community-discussion/real-preloader-on-a-wix-website
Or you can look at adding the Mobile Tool of Welcome Screen that Wix provide for mobile versions through the mobile editor itself.
https://support.wix.com/en/article/adding-a-welcome-screen-to-your-mobile-site
https://support.wix.com/en/article/customizing-your-welcome-screen-on-your-mobile-site
Plus, continuing on from Giri’s comments above, your mobile version will be slower, hence why the preloader or the welcome screen from Wix is a great solution.
However, you also have to take into consideration the actual mobile device and internet connection that is being used to view the website on too.
With desktop the user might have fibre or fast broadband ethernet connection at home so opens on pc or mac fast, whereas on mobile they might have an older phone or a slow wifi speed or might be on 3g still or poor 4g connectivity or a slower public wifi for example.
So that will be a major factor to make the website load slower and there is nothing that you can really do to make it faster.