It sounds like your area is probably outgrowing longfast. They actually wrote an entire blog post about this particular issue.

https://meshtastic.org/blog/why-your-mesh-should-switch-from-longfast/

Edit: meshcore sits between MediumSlow and MediumFast meshtastic presets. So 40 nodes are trying to use a preset that is slower than what mesh core was using for very few nodes.