One of the main themes over the last couple of chapters, has been performance. That's not an accident. Whether you're sending AJAX requests to the client from the server, or sending script files from the server to the client - whether you're executing synchronously, or asynchronously - it's important to always consider "what impact will this have on the user experience, can I improve that, and (if not), is it worth it?"
Often, a few-millisecond delay in processing is well worth the added or improved functionality that a given feature might add, but even so, it's important to consider whether there is a more efficient way to accomplish the same end-goal. "Lag", or non-responsiveness, is a cumulative frustration. Here are a few things to avoid, in order to maintain a positive user experience. These are not hard "rules" that must always be followed, but they are general guidelines that, if you go against, you should consider the reason, and if there is an alternative...