It is no secret that web requests take bandwidth and time. Many website hosting plans actually use server resource usage in their billing calculations or restrictions per plan. Are you looking at ways to reduce resource usage, even at the margins? Using redirect-less, or AJAX calls, can help with this, as your web servers will not have to go through a whole user appropriate front end page generation.
On a general level, this holds true. As a caveat, it is up to the plugin and theme developers to design their code in a way that ignores unnecessary execution on AJAX requests.
As you may be able to draw the lines yourself, not only do the requests take less resources, but overall the time spent on the request is shorter than doing the add to cart as a redirect.
In special cases, custom programming can be implemented to bypass much of the WordPress installation and other plugin or theme startup cost. If you are interested in this type of special custom programming, please reach out here!