the interface is filled with slowdown

You need to sign in again through the Xbox 360 interface assuming you're using Xbox One/Xbox Series, then it will let you log in. The game is filled with modders however so be warned.

They build in some slowdown CroutonCrusader, you'll need to use that

There will be a lot of turnout in the servers and this causes slowdowns

In my case it really was just the login interface not rendering. "Solved" it by using your print to PDF trick:

1) Right-click login interface (the white screen)
2) Click Print and save to PDF
3) Open the PDF to see what should be displayed
4) Fill in the relevant fields and repeat steps 1-3 as necessary.

These are ridiculous hoops to jump through, but at least I only had to do this once.

Crazy that we still have to deal with a login interface not rendering in this day and age lol

Xbox One is a console. It was designed to prevent the fragmentation you believe is happening. Performance slowdowns of the UI may simply be too many background processes open (something a cold boot fixes).

I haven't defragmented a volume since the days of Windows 98. There's no need to with NTFS if you don't abuse your HDDs by filling them to capacity. For SSDs, there's never a need to defragment (and it's bad for the underlying NAND).