For the speed issue; I was playing around with an idea of preloading the iframe previews to speedup loading; it worked but the browser would crash due to high CPU/memory usage eventually. Maybe you'll have better luck?
About the pre-loading, I'm actually already doing that... But only the next one in the queue. How many were you trying to load at once? I did notice that even only pre loading one is enough to slow it down considerably on my nexus 4 though...
BTW: I like your mobile/tablet previews. I was actually going to do the same but left it out for the moment :P
EDIT: Might be worth investigating the (dis)allow-scripts variable of the sandbox tag? Take a look at http://www.html5rocks.com/en/tutorials/security/sandboxed-if...