Switch default compressor to zstd
Due to small window size, gzip does not take full advantage of the inter-file correlations. All other modern compressors do better; however, only a few options are actually useful. xz is slow to compress and decompress; the latter is particularly bad during boot, as the 16-bit real mode kernel decompressor stub is even slower. zstd is a good option because it decompresses quickly. It can use excessive ram when compressing small files at very high compression levels, but for typical initramfs size (around 15 MB) and zstd -19, it uses only about 150 MB.
Implementing this would decrease the initramfs size by about 3 MB and speed up boot by about 0.1 seconds on a 3 GHz amd64 CPU; on a non-overclocked RPi Zero, that could be almost half a second of boot speed-up, plus the decreased I/O (another few hundreds of ms on a slow SD card).