Thanks for the review/feedback! You've identified two things that we've put a lot of work into at ShareLaTeX: Good real-time collaboration, and fast good PDF previews. Getting these right have been core to the whole product - when it works you don't really notice, but if it doesn't work perfectly, it's really frustrating. Collaboration and our scalable LaTeX rendering infrastructure are probably where most of the complexity and 'cleverness' in our code lies, so it's nice to know it's well received :)