When your area's mesh starts getting good they generally move to MF. While it's still sparse generally they'll stay on LF.

The Mesh is prone to overloading and MF helps with that while also being faster than LF.

Chick to see if there's a local Meshtastic group and what setting they recommend. https://meshtastic.org/docs/community/local-groups/

For example, the Bay Area mesh recommends MF. https://bayme.sh/docs/getting-started/recommended-settings/