Expensive? Yes. Slow? Doesn't matter. Merchants accept Bitcoin through third party payment processors, and for amounts below $50 they (assuming you add an appropriate network fee) will not require you to wait for confirmations. In easy terms, the only bother are the fees people have to pay, which in this case would make it not worth to use it for small purchases. If you look at the average amount people spend in McDonalds, it's around $10. If you include appropriate network fees, you'll pay like $10 for the food, and $5 in transaction fees. At least, there is not much to worry about since McDonalds isn't accepting Bitcoin.