Low FPS is often caused by too many instances (building a base, terraforming/etc) and the meadows is often where players choose to build. It sounds like you are experiencing what many already have.

There is a settings/file tweak that seemed to help my low FPS in the bad areas a bit. My AMD 5700XT got about a 30-50% framerate boost with this fix:

Right click Valheim in Steam library, click 'Manage' and 'Browse local files.'
Open the valheim_Data folder.
Right click 'boot.config' and [edit] with Notepad / Notepad++.
Add "gfx-enable-gfx-jobs=1" as a new line to the very top.
Add "gfx-enable-native-gfx-jobs=1" right below the first line.
Save and close the file.