At ScribTeX we regularly get comments from users about how fast our compiling is. Even with the round trip to the server, the PDF appears in the user's browser quicker than if they compile it locally. This isn't due to anything especially clever that we've done but instead stands as a testament to data speeds, and the performance of the Linodes that we compile on. Particularly with Linode, I think the high disk IO performance makes for a very quick LaTeX compile.