a very oppressive 5 second turn limit
I think that might be the problem. I can play chess... well, not fantastically, but alright, in general, but ask me to play bullet? Nuh-uh, no can do boss. Same for blitz, to a lesser extent.
In Yakuza 0, you can play online, and the turn limit there is 15 seconds, and I thought that was VERY fast, as there's quite a lot of things to look at and think about. You have your own hand, the tiles that have been discarded, the tiles that HAVEN'T been discarded, everyone's scores, what turn it is, who's the dealer, how far along in the game you are, how you want to shape up your hand, how you think your opponents hand might look (I can't do this at all), and heuristically measuring how likely it is that someone will deal into your hand or even complete it yourself to win.
When I'm playing against the AI, there's no time limit, so I'll just stare at my hand for a minute or more sometimes thinking of what I want to do with it. Obviously this is all new to me so I'll be slower than someone that grew up in a society that has this game ingrained into it, so maybe I should just take the lumps and suck it up. It's like the Ping Pong Club that I like going to when I can, everyone there has been playing the game since they were little, and perhaps several times a week, while I only discovered the game when I was maybe 22 or 23, and I almost never get to play at all, even though I'd like to. Just gotta make peace with the fact that I'm an idiot infant in the body of an almost 30 yr old man, and really the only thing I can do about it is to not be self-conscious about it.