This add-on only updates actively viewed threads, not total number of threads. I'm fairly sure most XF forums don't have nearly that many active threads at any given time.
The code actually works based on total number of keys, so it just takes longer to visit all of them;
Move some view counter to Redis-based increment counters rather than MySQL - XenForo2-RedisViewCounter/ContentView.php at 1dffbf1abd3586f49f51cc1fd2f8e910855b4088 · Xon/XenForo2-RedisViewCounter