For me, it seems to run just the opposite. I need to fire 10 or 15 shots to "warm up" the gun (me?) before my groups settle down. I've never timed myself, but I'd guess I fire 3 or 4 shots a minute, with a short pause between groups or to change targets. Once I get in the groove, things usually hold steady or improve slightly. After an hour or so, fatigue becomes a factor, at that point I either stop shooting, or just shoot at fun targets like charcoal briquettes at various ranges.
Dave