What is it that make extensible RAM inherently slower? Is it the connectors raised off the boards, adding physical distance to the traces that's the issue?
The mechanical connector layout forces sub-optimal signalling paths.
You're forced to make everything slower to make things consistent.
That being said, a more optimized connector, board layout and form factor could make large-capacity, lower-latency modules possible. Not sure for example if there is anything out there besides the Dell proposed CAMM form factor, and I don't remember that one being specifically promoted as "faster", mostly "smaller".
