An email came in last week asking just that question. The answer is a little more involved than you might think, because there are some variables involved that simply don't exist with the same action in an autoloader.
There are at least a half-dozen different ways that I've used to reload a revolver, and I've seen variations which exceed that number. Each technique has strong and weak points, and it's up to the shooter to decide of they fit his/her situation. For instance, it's possible to shave corners in technique which decrease the time required for the reload, but which increase the chance of failure (case under extractor jam, speedloader release binding, debris under the extractor, un-ejected case, and so on.)
There's also a big difference between using speedloaders and moonclips. The moonclips in and of themselves aren't all that much faster than, say, a Comp III or an SL Variant speedloader, but their all-in-one nature allows the shooter to cut those aforementioned corners without the associated risks. In my experience, using moonclips will shave .4 to perhaps .6 seconds off of the average person's reload times. In competition, that's a huge bonus over the length of a match. In self defense? I personally wouldn't carry a moonclip revolver for self defense, my rationale having been well documented in this blog and elsewhere.
All that being said, if you want to see what's possible when all the conditions are perfect (talented shooter, moonclipped gun, and lots of practice), check out the famous Jerry Miculek video:
Back here on earth, I'll share with you my personal experience. When I was shooting competition very regularly and thus "in shape", my average time with Comp II speedloaders was something in the 2.8 second range. A Comp II loader would typically cut that by only a tenth or so (I found the much larger Comp III to be harder to handle in my tiny mitts, which reduced their speed advantage over the Comp II. Most people do a little better than that.) When the stars were aligned and I was having a good day I could do noticeably better, having hit 2.5 seconds in competition more than once.
My considered opinion is that anything under three seconds using speedloaders is pretty darned good; most people can't do that with an autoloader!
My very fastest reload using speedloaders, and one which to this day I can scarcely believe, happened during a Steel Challenge-type match about a decade ago. I'd missed one target before I got to the stop plate, which means I had no room for error. If you've shot SC type matches you know what happened next: I missed the stop plate! I could tell as the shot broke that it wasn't going to be a hit (again, steel shooters know that feeling) and immediately started a reload. I hit the stop plate with round #7.
The guy holding the timer, who'd himself switched from revolvers to autoloaders some months prior, looked at the timer and said "If I could do that I'd still be shooting the wheelgun!" There on the display were my seven shots, and the split between #6 and #7 was 1.98 seconds. The gun was a Dan Wesson Model 15-2, the speedloader was a well-worn Safariland Comp II, and the bullet was a LaserCast 158gn SWC.
I don't remember it seeming all that fast; I do recall it seeming to be effortless. Never before or since, no matter how much I practiced, was I able to recreate the occurrence. In fact I haven't even come close, which leads me to consider the possibility that it might have been some sort of timer malfunction. If not, it shows what is possible under the right conditions.