I notice when I start playing shiritori early in the day, I get loads of xp. Typically, I’ll raise a level. However, by the end of the day, I seem to get nothing. For example, right now I am sitting at level 91 with 0% growth. I must have played at least 30 Shiritori words since hitting level 91. Is there a cap on how much xp one can earn through the games every 24h?
It's practically impossible to get more than an average of 2 (give or take 0.2-0.3) levels a day. This is assuming consistency.
All quiz related activities have shared diminishing returns, while games and community activities (including XP from forum posts) have their own shared DR. For the latter it's around 1 level a day.
There is a hard cap on points - as the last person noted, it's basically impossible to get more than 2 levels a day. Part of those two levels comes from studying, part comes from community/games stuff.
It's meant to encourage consistent studying over cramming a bunch of work into a single day.