ajax – How can webform performance be improved on low internet connection?

I built a site with Drupal 9 and webform.
The form consists of three pages and contains a few computed fields.
On the first page are mandatory fields.

When on a machine with a normal/fast internet connection, everything is fine.
When on a machine (smartphone) with a slow internet connection (or on „Low-end mobile“ in chromium’s developer tools), I can enter all information on the first page, press next, it loads the second page and jumps back to the first one, claiming that the mandatory fields were empty.

Is there something that should be cached and that I missed so far? Do I have to manipulate some timeouts (where are they, I didn’t find them)?