- Annoyance
The current unraid default when you enable compression on btrfs in 6.12 is compress=zstd. (can be confirmed with mtab). However, this is generally not recommended by most in the btrfs community.
Instead, compress-force should be used instead. The reason is, with compress, if the first 64KiB of a file is not compressible, Btrfs will not even attempt to compress the rest of the data. Compress-force meanwhile will attempt to compress the entire file, even if the first 64KiB isn't compressible (which can definitely be the case for many files).
Using compress-force wont have any noticeable impact on performance, and in no case will it use more disk space. ZSTD compression is blazing fast, and any data that isn't compressible will be discarded anyway.
Doing this will significantly increase compression ratios. Benchmarks a few years ago show default level 3 compresion, which Unraid is using will achieve over 800MiB/s on a Xeon E3 1650, so only on NVMEs would it potentially be a bottleneck. That's a rather old CPU as well, newer CPUs will be even faster.