Well I am not yet sure why, just try Fxp 115 cm9 for yourself..
It runs very fast with low memory footprint.
Also if u look at system app, u will notice that all apps take less ram with FXP.
Looks like cm9 is a lot better optimized than official Sony beta rom.
Benchmarks seem to confirm it:
Quadrant 1650
NenaMark1 60.4
I have used CM9 for enough time...That is witout any question a very good work and I have big expectations from that...
BUT I am sorry but the choppiness existed in CM9 every moment.. its still not perfect... There was Lag free Opening of app drawer but that too gets enough laggy when u install google apps..
My phone didnt deep sleep for a sec... battery drained like 10%/hr without having even those google apps installed.. it might have features but still not perfect... I would also wish for that perfect port for them coz I like AOSP builds myself free from Sony bloatwares...
Let me assure, u will not continue using CM9 for many days...

.. its still not perfect to be used as daily driver...
anyway, There is work going on with AOKP Kang build, Hope I get that GSM fixed then get ready to feel the smoothness... 240mb Ram free was what I was getting...
If you know enough dev, I would be glad to share the build with u to resolve the issues coz thats just mind blowing...