I've been in a friends-with-benefits situation for the better part of a year. I wasn't the one who suggested it, and I was initially reluctant, but it's been pretty awesome. This is very different from unattached casual sex, mind you. We're very good friends, we care about each other and so forth. But we've been able to keep this up long term with no romantic attachment at all. Technically we're both free to see other people, but since we're both busy and pretty satisfied with the arrangement that hasn't happened I guess.

I agree with another commenter that a relationship would increase your productivity, but it's a little more subtle than that. When you start dating someone, initially it's going to be a huge time sink, but once you get over that hump and you're in a stable relationship it's probably going to leave you better off than being single. But you have to accept that a significant initial effort is needed into building the relationship and also accept the risk of breaking up and having to deal with getting over them, and so on. Nevertheless, I'd rather be in a committed relationship; but if work productivity is the only criterion, my current situation works great.

So here's my "solution": find someone who's as busy with work as you are, and find a way to be in a low-key, long-term relationship. Long-term is important; trying to find someone to sleep with every once in a while is probably the most time-wasting strategy of all. (Never tried it.)