Map size alone shouldn't affect performance very much, when I test an empty 50x50 map and an empty 256x256 map there is not a noticeable difference in performance. I think when people say this, it comes from the fact that you will probably have many more events on a 256x256 map than you will on a smaller map, which this plugin helps with.
By default, pathfinding searches up to a 12 tile distance, this plugin lets you change that, if you search less tiles it will perform faster because there is less to check with trade off being it will not be as smart.