Ubuntu for phones already got a performance update (video)
The video comes from Mika Meskanen, an Ubuntu interaction designer, showing off two devices (both look to be Galaxy Nexus smartphones) - one running the original build of Ubuntu for phones and the other running the new update. One of the things that we had all noticed from the early hands-on with Ubuntu for phones was that there was some lag and stutters in the UI when scrolling and such. From the new video, it seems those issues have been smoothed out.
It is just a guess at this time, but the assumption is that performance updates like this are why the test images have been pushed back to a late February release, rather than "in the coming weeks" as was originally promised.