When i was playing splatfest it only took a few seconds extra to find games. Might be because a lot of people are active during splatfest, but i'd still love to play regional matchmaking.
Splatfests are events out of the ordinary. They take place during the weekends, which is naturally when more players are available, and by their very nature they attract more players than usual. Splatfests are far from being representative of what would happen during valley hours in player count when you include regional matchmaking.
Besides, regional matchmaking would divide the playerbase in more ways. Japanese players are numerous and close to each other, so it's highly likely a huge number of them would stick to regional since it'd just be so much better, thus taking a huge chunk of global players out of the picture.
Europe is also relatively close knit, but there are less overall players. It's likely regional matchmaking would work like a charm... when it actually manages to find enough players in reasonable timeframes, that is.
America... oh boy, America. That place is huge, and with a particularly high concentration of potato connections (dumb monopolies in the US, a respectable number less developed countries in Central and South America). As I said, Splatfests have been the times I've faced my most ridiculous cases of lag during my playtime, and without a really significant increase in game performace. You would now have regional matches with the same ocassional hickups and significant downtimes, or global matchmaking where a good chunk of the playerbase is sticking to their regional matchmaking. Not a good experience overall.
