This is the best overview of spaced repetition research I have seen online or offline. It's thorough, well-argued and probably covers more than you were looking for. It's not specifically about Chinese, of course, but is highly relevant for anyone who wants to learn anything.

This is from the introduction:

Spaced repetition is a technique for efficient memorization & practice of skills where instead of doing a lot of work quickly, each item’s practices are automatically spread out over time, with increasing durations as one learns the item. I review what this technique is useful for, some of the large research literature on it and the testing effect (up to ~2013, primarily), the available software tools and use patterns, and miscellaneous ideas & observations on it.