I'm not going to even to attempt to explain this one. Guy Robinson's blog Bolt Out of the Red has a new post yesterday that does. He's a programmer whose been specializing in Revit API projects for about as long as a person could be and not be a Autodesk employee.
I've responded to posts many times at AUGI asking this question or insinuating that Revit is slack for not doing this already. I think it is a much more important request to ask that performance overall be improved rather than state that the solution we want is "multi-threading" as if that alone solves the problem. I don't have the brains to even pretend to tell them how to solve it, I just want more, fasterer and betterer...soonererer. Thanks in advance for more, fasterer and bettererer.
Thanks Guy for your attempt to explain it!