See the release notes on Mx7.11
They are aware of the issue. Some info from somebody at Mendix would be usefull on what is causing this though.
Regards,
Ronald
A modeler issue introduced in Mendix 7.11 caused the low performance of the 7.11 Desktop Modeler for large apps.
This was not related to newly introduced features nor the running app.
We've solved it in Mendix 7.12, so please use this version instead.