The user already is. The casual user's system is idling with all sorts of nonsense. Adding some light processing doesn't harm. I'm thinking 100-500ms per page. You don't render the page for the user neither, do you?
For heavier use cases (e.g. image processing), the user should be willing to spend some CPU power. It doesn't make sense to send an image to a server, put it in a queue, wait for an image processing worker to run it, and send back the result. It's simpler and more sensible to run that process client-side, if feasible. E.g. LLMs are too big for that, but many other tasks can.
