> Users report that my web app is sometimes slow.

One wise move is to validate that your app really is slow. Do you experience the same slowdowns? If not, it could be network latency, which is completely dependent on where your ends users are working.

I work with many rural end users, and we have noticed that specific areas of the country tend to be where we get complaints of slow apps. We've worked with ISPs in that area to identify and correct old hardware on their network, which improved performance of our app.

If you can reproduce the slow behavior, by all means, follow everyone else's advice. But just like you would reproduce a bug before fixing it, reproduce the slowness.