in my experience the entity count has a major factor to fps on server when it coms to lower server specs . instead of lowering map size . first try this .
spawn.max_density 0.5
spawn.min_density 0.5
in server console and follow up with
writecfg
also make sure the top is also included in your settings.cfg
then after restart you should load less entitys (animals ,ores, trees and collectables)
as i see it with lower pop servers you dont need so much resources ... and this (atleast for us) jumpt our server fps from 50-60 up to 100 - 120.
hope this helps