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Good question. There's not much we can do at the LaTeX level, but writeLaTeX also has to send images of the pages back to browser, and we can improve that part of the process in quite a few ways.
Perhaps it would help to checkpoint the compilation periodically, keyed by hash of input consumed so far, so when adding a word on page 100 you don't have to load all packages and recompile 100 pages. - One approach is fork() processes (the tricky part being flushing all I/O and redirecting to separate files). This only works in RAM, which means active users & machine affinity, but I suspect storing checkpoints for l…
Thanks for the suggestions! The fork() idea is very interesting --- I hadn't thought of that. Drop me a line at john at writelatex.com if you ever want to chat about tex stuff. :)