In parts (1) and (2) comparing the default setting of Zstd (level 3) against the default setting of Brotli (level 11) is a bit misleading. It shows Brotli compressing ~30% better than Zstd, but Brotli's default level is >100x slower than Zstd's default level. Zstd level 3 is expected to run at hundreds of MB/s, and Brotli level 11 is expected to run at ~2 MB/s. The compression speed is only 30% slower because that benchmark includes the time to tar the directory, which is likely more expensive than the compression itself. As @sfink already suggested, just running lzbench on the npm-9.7.1.tar would be a better benchmark.

In part (3), because its running only on lib/npm.js which is 13KB, you are getting skewed results which aren't directly applicable to the compression of npm-9.7.1.tar. Brotli excels at compressing small Javascript files, as this is where its dictionary provides the most benefit. The benefits of the dictionary for a large tar file will be negligible.

However, in the npm-9.7.1.tar scenario we still expect Brotli level 11 to produce slightly smaller files than Zstd level 19. Likely ~5% smaller. But we do expect Zstd to provide significantly faster decompression speed.