I don’t think GDevelop is made for open worlds games, I mean those games have dynamic memory management to load/unload resources as the player moves in/out around zones. So I think you’ll have to do it manually, saving dynamic objects in external files to load and create them when needed, and creating static objects from external layouts, for example the terrain, but not sure about the overal performance 
What caught my attention is the freeze time in the compiled game that doesn’t happen in the preview, because I have already readed this some time ago: [url]Compiled game take pauses]