I recently upgraded my Ubuntu version from 19.10 (Eoan) to 20.04 (Fossa) on my XPS 13, which now runs Fossa on Kernel version 5.4.0-31. Fossa detects the Wifi card, and connects to my home network, but internet connection is very slow over Wifi.
Eoan didn't use to detect the Wifi card, so I had to install the backport-iwl-wifi on a slightly downgraded kernel: https://askubuntu.com/questions/1046589/backport-for-iwlwifi
That fix doesn't apply here.
Also removing the backports iwl package: https://askubuntu.com/questions/1230525/ubuntu-20-04-network-performance-extremely-slow didn't work.
Does anyone have a solution?
Yes, disable the intel wifi driver in the "Additional Drivers" section of "Software & Updates." This worked on my Precision 5540, should work on your XPS 13.