Lots of players play games sequentially. At the very least, you probably get a timestamp of when games end.
If two games ended less than say, one hour apart (maybe need some tweaking here), they were likely played back to back. Then you have a rough idea about a particular games duration.
You probably have hundreds of thousands of back to back games by players.
From this, you can try to build a model of player slowness from the game by game durations.
Yes, yes, yes, I understand that this is noisy, that the f'ing minion secret chamber game will just take forever and it's not your fault, that sometimes you grab a snack or make a telephone call before starting your next game, that sometimes the games are slow because of the other players, etc. The data isn't going to perfect, the model certainly won't be perfect, but it is still plausibly useful.
Bonus points: Make the goko extension grab the player speed data and display it before the match starts.