I’ve found the reason and a workaround:
Disk I/O. More specifically mostly the writes to the Saved/ folder. The game somehow constantly (and in my opinion unecesssarily) updates the .db files in that directory. Probably to always have a recent safe file/backup in case of crashes or similar.
Workaround: Create a ram disk for that folder or get a fast ssd/nvm drive. With a ram disk you need to make sure to make backups of that folder every so often to aviod the loss of your progress. Funcom should have made most reads and writes in ram and only do (blocking) i/o every few minutes.
The question is why this problem gets worse with placables. You shouldn’t have to write the coordinates of static objects to a file constantly. Maybe it has to do with the items inside those buildings that will change due to crafting and decay. Even so… the i/o is way to agressive and could be toned down a lot. Losing a few minutes of gameplay shouldn’t be too terrible if it doesn’t happen all the time?