Well, I can tell you that they are pretty optimized :), I have already get rid of the most "lag" feeling ones
But yes you can optimize them even more, in fact I think that the "black and white" E17 theme looks pretty more lighter! try it
And also the default theme, can be a little faster (or maybe not)
Bling bling is also much older and should be pretty fast too, try them all!
If you are in elive 3.0 you should be able to install / use the themes directly by opening them in the filemanager, there's a smart elive tool that manages them
There's MANY amazing themes you can try for E17, but is better to search about them in the Themes category , specifically the black-and-white one is here:
, specifically the black-and-white one is here:
