There's so much that could impact performance - how much work you can get the graphics chip to do is one, where you're exporting to is another. I've seen odd behaviour on different systems and between the different point releases of 11.x. Any v9 model will open fine in v11, so there's no harm in using v9 to edit models if that's quicker. You can't use Cleared By, which handles so many complex report layouts, and you're limited to fewer Append and Footer templates, and there are fewer PDF import options, so watch out if your models make use of these 11.x features.
I'd be delighted to have a chat about this, and bounce some ideas around about how Monarch could be improved. Please drop me an email if you'd be interested.