Is your full landscape loaded? I mean in world partition are all the cells loaded? If so, that would be a large demand on resources at once. So right click and unload all cells, then just highlight the few around where you are working and load them.
I had similar issues when adding the ocean around it too, got a lot of out of memory issues and I was like wtf can’t be.
I recently captured what I have learned for my own benefit and placed on youtube.
It is in three parts, and goes through some issues in real time LOL!
Hope it helps you.