One game a day, the same for everyone — build a streak in five minutes
One game is chosen each day and everyone plays the same one. It takes a few minutes, it builds your streak, and because the whole world is on the same challenge the daily leaderboard is an actual like-for-like comparison rather than a mix of games.
Spacing is the single best-supported principle in learning research. Five minutes on six days produces more durable improvement than thirty minutes on one, for essentially every task on this site.
Short sessions also avoid the fatigue problem. Accuracy on memory and attention tasks falls off sharply after about twenty minutes, and a tired run teaches sloppy habits rather than good ones.
A rotating daily game also stops you specialising. Playing only what you are already good at is the commonest way people plateau.
Completing the daily game extends your streak. Missing a day ends it, and nothing is taken away when that happens — no lost XP, no locked features, no notification chasing you back.
The streak lives in your own browser, so it does not follow you between devices.