> Since the old version of the game is known on both sides, we compress the new version using the old version as its dictionary.

That's quite clever!

> Since we compress once and decompress many times on player machines, we can afford slow compression times. Zstd lets you tune the compression level, and we found that level 19 yielded about 13% better compression than zip.

Zstd is parallelizable across threads, which wasn't mentioned here. It helps speed it up at high compression levels, though not as much as I'd like.