The latest Rhino WIP (May 16, 2017) contains a large rewrite of the block display code. You should see performance improvements in models that use many blocks (including nested blocks.)
This was a large rewrite so I won’t be surprised if there is a bug or two that still need to be squished. Please let me know if you have models that are not showing any performance improvements or don’t appear to be displaying blocks correctly.
There is one outstanding bug that I already know about where blocks are not properly displaying transparent (ghosted) surfaces. https://mcneel.myjetbrains.com/youtrack/issue/RH-39146