It seems that the SystemUI has a memory leak, and starts consuming an abnormal quantity of RAM. This means that apps will start to be killed and relaunched, including the UI. The consequence is the sluggishness of the phone. Rebooting solves the problem, but it's only temporary.
One suggestion given in the RootzWiki forum was to disable the camera widget in the lockscreen. This seems to not work for some people, but it worked for me.
Just download the Lockscreen Policy app, and disable the camera widget (or all the widgets completely) from the lockscreen.
Other widgets you may may have included in the lockscreen may also be the reason for this low RAM problems.
In the RootzWiki forum, another option emerged: a unofficial AOKP v4.2.2 based version.
I've been running the Feb-20 version for some days with no problems.