If you want to try this for yourself, here are the steps:
On your Steam Deck, go to desktop mode and download the file from this website. That file is a config for lower than lowest graphics.
Next, open dolphin and navigate over to "/home/deck/.local/share/Steam/steamapps/compatdata/553850/pfx/drive_c/users/steamuser/AppData/Roaming/Arrowhead/Helldivers2". Once there, make a copy of your unaltered user_settings.config, store it somewhere like your desktop (thats where I do at least).
After that, just drag-n-drop the downloaded file into your directory. Don't boot the game just yet, if you do, it will alter the new settings to default. To fix this you should make the file immutable, that way the game cannot change any settings and you can play normally.
To do this open terminal in this directory and type in "sudo chattr +i user_settings.config" and put in your password. If you want to undo this, just put a -i instead of a +i and run the command again.
At this point you should be able to run the game with the changes, the game won't look as pretty but it will run a helluva lot faster! Thanks for reading, have fun killing bugs!
Found all this information from this reddit post, thank you to u/MattyXarope for the file!