Mine is not laggy at all....
bcoz I've hacked it and disabled the Nokia Lifeblog....
Need to wait a for couple of seconds when opening gallery....
other apps and programs, is just fine...just like using a S40 lor...
1st, need to hack(or in other words install ROMPatcher)...
2nd, install Jbak Taskman...
3rd, follow some instruction to disable the lifeblog....
I think it can be done under 30min only....
really worth the effort...

And yes...themes can effect Ur phone performance....
But not so noticeable la....
If U use some fully vector themes, it will make the phone laggy a little...
There is a 'basic' themes that IPmart user created to speed up the phone a a lil bit...
the theme is jut like Nokia 3650/N-Gage UI....simply white background and default icon...
Will try to search for it if anyone here want to use it...

Erm.. sorry for being noob, but where can I get ROMPatcher and Jbak Taskman?
My theme is quite basic, which is just change the background, no font or icons changes.