Did you try to give more memory to Data Modeler?
Edit file C:\Users\UNAME\AppData\Roaming\sqldeveloper\126.96.36.199.0\product.conf
Find line AddVMOption -Xmx... and modify it to:
AddVMOption -Xmx4000M or to value that works for you.
Check also this - Re: Memory Problem with Big Model
I've changed the setting as following:
My machine has 3.5 Gb of memory.
While opening the model, I opened the task manager and saw that cpu usage was 50% and max memory was 755 Mb. Whne performing some actions the memory raises to almost 1Gb. So certainly, your response helps.
Once the model was opened, the cpu usage dropped. Clicking on a node was this slow. Then I closed the navigator window and performance is now okay. It seems like, when first opening the model, the navigator shows the diagram with all entities in it.
To avoid this, I've turned of the "visible" property on the "Logical Model" node.
Thanks for your help.