For normal servers:
1) Use Fast Async WorldEdit (FAWE) instead of WorldEdit. This should seriously help.
2) Check your server view distance. This is in server.properties. Higher view distances generate exponentially more chunks - a view distance of 40 can result in 5000 chunks loaded per player.

For low-end servers:
2) Make sure you have enough RAM. It is recommended to have about 3gb of RAM or more to run the plugin smoothly (the plugin doesn't use 3gb, this includes what Minecraft and other plugins need).
3) Alternatively, just pregenerate your world! There's a lot of plugins that allow you to generate your world, and as long as BetterStructures is installed when you do that you will see the structures. Once generated BetterStructures will not use any power! The only downside is that you will not be able to download more structure packs without generating a new world, meaning that whatever you had when you pregenerated the world is final for that world, same as with vanilla Minecraft!