Performance: Nested Blocks versus Groups. Any changes in R9?

I’ve read on this forum that Rhino doesn’t like nested blocks? Is that true? And if it is can you still get away with on level of nesting?

Right now I use groups instead of nested blocks. It does start to cause lag at a point, so I’m wondering which is the lesser evil.

Generally I can get away with one layer of nested blocks I don’t think I’d go deeper than that, but I would have a lot of nested blocks.