If I do not the Repo feature, will this mean that serving the page with the products on will be slow for visitors at all?

Not necessarily. ASA 2 offers some caching techniques (see http://docs.getasa2.com/caching.html) and an AJAX mode (see http://docs.getasa2.com/options_ajax.html), so the page will load fast without using the Repo feature and the cronjob.