"Months" was a true statement. It was with an old Intel 430x chipset [I think that's correct] and the "bug" if you loaded more than 64 megs of memory. They would go to "never neverland", and took a long time to come back.
[As to the boxes]
Each model is designed by the manufacturer. The "how they do that", is up to them, so they don't all share a "standard architecture". Similar yes, but not exactly the same.
It's clear that the HR20 has a better processor than the newer boxes. "Most" changes in the newer boxes have been to lower costs.
In a Chat with DirecTV, "they" know about the slower speed of the HR21/22/23 and are hoping to improve this in future releases.