In the case of the kobo, it is firstly a much more open platform than kindle. Secondly, existing projects have made much more performant reader apps than the stock one. Basic kobo (nickel) is a bit slow and can be even slower with a “bad” EPUB file (you will come across many online as they are often auto generated from other sources and are essentially a collection of html files). Installing a third party reader can make page flips and re-renders significantly faster and smoother as well as remove all unnecessary network processes