Just because you're working out with a friend doesn't guarantee efficiency. Oftentimes you'll start talking and get distracted. One solution to this is the buddy workout routine. You do one set while your friend rests, then you take your rest and your friend does their set. There should be no instance when you're both resting – that's what you're trying to avoid. You don't have to particularly arrange your sets so that they match up. Just make sure there's no conflict in the weights you need (if you're using weights).
Also, now if you're being lazy or slow, then you're answerable to your partner. Not only are you not working out but ruining their workout too.