It's called blob mechanic. If greater then 10 players on a map they turn into a bright red dot.EDIT: Keep in mind when I say a movement speed debuff in a group of 10-20 people, I don't mean an actual ingame "Group". I mean 10-20 people on the same screen as each other occupying a certain area of space. The more people within the same X distance as each other, the slower they should move.
So hows this going to work? If your in a party of 10 people you get a slow buff, so keep parties smaller? If your in the same alliance/guild and in a group you get a slow buff? Bye bye raid zones that sometimes have 10-20 people, castles take 20+ why should they get penalized? If the blob gets closer to you is it going to inherently slow you as well because now your in the blob. This idea is garbo no offense.
SBI has to do other things to the game then penalize large groups. Better activities for large groups to bash heads against. Currently the only activity is castles, warcamps, city plot that provide 20+ groups any activities. World bosses don't even take 10 people now.
Even if you added a huge pvp dungeon that funneled mobs down moba style where it required a big group of 20+ to separate into groups of 5 would be better then some of the shit dungeons they have now.
OR bring in the Ultima Online Champion Spawn. Open world where you have to kill certain spawn over and over with progressively more difficult mobs (scaling hp with more people in the area) when you kill enough a champion mob spawns and when killed drops some good shit.
And yes, if you a blob were to get on top of you then you would also suffer the same penalty. The entire point of it is that being in close proximity to other players outside of cities/safe zones should give everyone an incentive not to stack up on each other. And if you do stack up you're punished for it.
One major drawback that might be unacceptable is that it encourages people not to use Melee weapons as much. So maybe this isn't so desirable after all.