Well....if you use ditto.pub or poster.place, it fetches TO/FROM many relays for you. If you use many relays on your client, it will use more data/power/battery to download from all those relays and process the data.

If you can use a super relay like ditto.pub or poster.place (spam-free ditto), you will be better off especially on mobile. Most relay admins do not do a good job fighting spam for their users like I do so using many relays will make you see all the spam