Like Rainisto said...:
"Usually its slow after the update, because its rebuilding its indexes, and that might take quite long if there is lot of media files or emails. Few days in the worst cases."

...it depends of how much data You have, to build indexes. You can notice that everytime when You add great amount of data to N9; movies, music, etc. It takes time and if indexing is dropped before finished, i assume, it takes more time... Be patient...

In my case, it was about 2 hours of sudden lagg. After that (+few reboots when installing gtalk-video+firefox+flash) PR1.2 has been _very_ smooth and battery last longer than PR1.1. Even #486 does not bother any more...