Speed doesn't matter.... what matters is how accurate and comfortable you are at that range. Add to that the fact that shooting at a animal in the field has many more potential problems than shooting at a target. Wind, twigs, animal movement, ect..

All in all, it is up to you. In the past, 20 yards was the furthest I would shoot, and now I would feel comfortable (given the right circumstances) to shoot out to 30-35 yards! I can stack them in their pretty good at 50 yards, but in my eyes, there are way too many things that can turn a good shot to a bad shot in the field at that distance!