1) How much it takes for you to level is based on a 7-ish day moving average. Someone who studies 100 terms a day levels at the same rate as someone who studies ten thousand.
2) On average, you gain no more than 2 levels a day (this is averaged over a long period of time). This includes hitting "soft cap" on games/forum XP. Shiritori being the fastest way to hit that. From my experience, XP you gain from quizzing and XP you gain from "games" is on separate DR, or in separate "buckets" (to use a gaming term).
3) Roughly speaking, a "soft cap" is a mathematical limit (usually not a hard limit). Both quizzing and games/community activities have it.
[ mostly copied from one of the many other times I've explained it ]