Hearing this makes me unhappy. I want the Modeler to be a fast and stable product for everyone. Of course, if models become really, really large, there will be a slowdown, but stability should not suffer.
I will look into performance with large models and I will personally fight for 64-bit support in the Modeler.
Do you only experience these issues when working with large models?
If you visit our office, I would love to see the problems you describe in person.