I’m not the first post to complain about how much the grid slows down the viewport. I’m assuming you’ve stopped looking at things like this in terms of improving V5 but anyway here goes.
Simple things that could alleviate the problem
In my case anyway I’m almost never working in all quadrants of the coordinate system, or not equally anyway. With something like a boat where the stern is 0, and it’s way longer than tall/deep, Rhino could easily be drawing 1/8 as many grid squares and still provide the functionality I need. Set up limits for each direction in the options instead of just one box for a number.
Obviously stop drawing tiny 1mm squares when you’re zoomed back enough that you can’t see them anymore.
In the mean time, have a toggle option in the Grid command so at least you can turn it on and off with a single button or hotkey etc.