Shooting fast mode with a bag gun, group match...
I consider 12-14 seconds for 5 rounds is fast. Less than 12 is exceptional. 15 or more is not really fast.
The more important thing, imo, is how to make a good group in fast mode shooting, not just how to shoot the string fast. BIG difference!
It needs a "perfect" setup & skill to be able to shoot fast. Lots of practice, good "ratio" between bullet weight/caliber & the weight of the rifle (such as PPC with 66-68 gr pills in LV/HV gun),... extra slick action-bolt lift-extractor etc,...correct action configuration for your best shooting style,... stock that tracks wonderfully,....extremely solid-stable-smooth front & rear rest,.... two eyes open (one watching the condition/wind-mirage changes) and extra reactive mind/brain of course,.... also fingers such as a magician's. Also a good/comfort position -correct chair height etc. Also a cartridge tray system.
I don't think you can shoot real fast with big cal/bullet in a very lightweight rifle. (I don't think you can shoot real fast with a skinny hunting style-round stock in bench shooting, just for instance). I don't think you can shoot real fast with unstable/lightweight/not slick front rest & rear bag. I don't think you can shoot real fast without foreend stopper. I don't think you can shoot real fast if you have stiff fingers. I don't think you can shoot real fast without a lot of practice. Etc.............
Like Jerry says it needs some luck, too. It's very easy to make a bad group in a very switchy condition using fast mode shooting. You certainly need to know when to start the string. You need to know/to learn the wind pattern of the range. You need to know when to not doubling.
Mike Ratigan is real good in this thing. If you look him shoots, he doesn't seem to shoot the string in a hurry at all but he can shoot the string real fast!
Why? He has all of that things above, and maybe more!
Does he always shoot in fast mode? I don't think so. It's all depend on the condition, and your confidence in that condition.
seb.