Yeah it's a bit weird because you'd think the node-webkit preview would work the same as the exported version since it's running a node-webkit instance when you do preview.
I'm not entirely sure if it's resolution myself. We increased the resolution from 640x480 to 854x480 (a 16:9 resolution) and it only impacted performance a tad because of our beta level stress test enemies since there were more of them on screen.
digitalsoapbox That's very impressive that you pulled that off, I'm not going to lie. Maybe our best bet would be to make use of Canvas since I've made a ridiculous amount of effort along with the other programmer to turn off everything off screen.