Ever seen a Farquarson?
In my opinion, they are one of the most elegant firearms ever created for sporting. You have no backup shot. It has been imperically proven that men shoot better at game when armed with a single shot because the know they have no backup. Plus, you show up in camp with one of these and prove you know how to shoot it, the PH will be more than thrilled to take you on safari. The Ruger No.1 is almost verbatim the same rifle. Functionally almost identical. Takes a scope with much less workaround than a majority of doubles, no need to regulate it. Just sight it in and hunt. I keep looking at a No. 1 in .458 Win mag with the intention of converting it to .450 NE. I just haven't gotten around to it yet. A successful hunter with a single shot is, in my eyes at least, the apex of the human predator.
All that said, wading into a pride of lions with one may not be the brightest idea, but I am sure it's been done. However, with practice, I would say it can be as fast as a bolt gun.
here is what Dr. Kevin Roberston a.k.a. "Doctari" has to say on the matter:
http://www.sportsafield.com/content/single-shots-dangerous-game
Notice the man who ran the course the fastest was running not a bolt, not a double, but a Ruger No. 1