I've recently been having this problem in the Galaxy forge where, once a substantial amount of planets are on the screen, 70-90, the editor begins hanging on every single action. Meaning that you can click on a planet, wait 5 seconds for the selection ring to appear, right click, wait another 5 seconds, click on "add phase lane", wait another few seconds for the cursor to change, and then click on a planet and wait a little longer for it to show up. And as time goes on, this gets worse and worse.

I'm currently running the Distant Stars add-on for the Galaxy forge, and my computer's specs run Sins just fine.