[369548684892826]

For example, to make some numbers up, if only the top 10 posts in each subreddit are cached, in normal circumstances the Popular page can be constructed from cache. If the number of open subreddits reduces by 95% then suddenly a request for Popular might be hitting the database a lot more.

Whatever happened is going to be more complicated than that, but I can believe people leaving, and subreddits going dark, could cause a crash.

[Nathan2055]

Pretty much a week after starting any web development job, you discover that 95% of making a site fast is simply reducing the number of times it has to call the database.

Typically, this means caching everything as much as you possibly can.

This is the same reason why Wikipedia can’t just roll out a cookie-based “revert to old look” option for logged out users. It would require caching every page twice since you’d need a file prepared for both skins. The reason they can have skins for logged in users is because they already bypass the cache for logged in users (to allow for individual user scripts and stylesheets to be run, among other things). Even then, things like article text and category lists are still stored in various database caches, hence why there’s a “Purge cache” option available to logged in users.