My last experiment with CSG is that it took a large of memory to intersect 2 meshes which crashed my browser ( roughly 4 GB limitation) when I had 551610 vertices merged with 138 vertices.
With 2 geometries of 22176 vertices and 108 vertices, the browser hanged for 8s.
So if you have the same things, my thinking is to pass it to a web worker or to process on the server side.