I'd resemble that remark, except I never set foot in Starbucks.
My point of view, being an "Internet cafe" is part of their business model. Seems rude to me, to try to annoy their other customers, even if you don't approve of how much (or how little) they buy, or how long they hang around.
BTW, if you get a phone app that downloads messages, instead of using (shudder) web mail, you won't care how slow the WiFi is. I've been using one called K-9, because it nicely supports having multiple email accounts (including a mix of gmail and POP3).