We heavily investigated zstd and met with the brilliant inventor, Yann, who provided amazing insights into the design and rationale behind zstd and why it is so fast and such an amazing technology. I also recompiled zstd into rust using https://github.com/immunant/c2rust and tried using various webasm mechanisms to run it (I didn't get webasm quite fast enough, and teaching c2rust to make it safe would be quite a slog).

But the main reason we settled on Brotli was the second order context modeling, which makes a substantial difference in the final size of files stored on Dropbox (several percent on average as I recall, with some files getting much, much smaller). And for the storage of files, especially cold files, every percent improvement imparts a cost savings.

Also, widespread in-browser support of Brotli makes it possible for us to serve the dropbox files directly to browsers in the future (especially since they are concatenatable). Zstd browser support isn't at the same level today.