Is there an optimum zerg size?
I’m not the 24/7 zerger some are, but I will spend a hour here or there following along. I have been in a few where they were so big that you hardly got a chance to hit the champ before he was dead and since you did little or no damage you don’t get the loot. Then speed is a problem as some toons go faster than others. But the running zergs are not so hard to do. The Way Point ones get lagged when so many players are all going to the same place. Usually, to solve this slowness problem In Frostgorge I will do the first three starting with the fish. Then I will skip the worm as I almost never get there in time and just go to the drake. Then there is usually a nice Mesmer who has put a portal up at the troll and I can get there when he’s about half gone. If no Mesmer I’ll just skip the troll and go wait at the fish. This way I’ll get 4 of 6 and get to breath too.
During the invasions I came to notice that 12-15 was my sweet spot for those events.
Northern Shiverpeaks
According to Anet there are 2 optimal group sizes for a zerg. A party of Five, or everybody on the map, and this is why we only have party sizes of 5, or 100g commander tags.
Nothing else matters.