I have lowered the level of "ZSTD" to get equal time with "FastBackup" and have the same result of compression, if no less in favor of "ZSTD".
Logically if I used the level "ZSTD -21" was obtained a longer processing time of about ten seconds, but a greater compression threshold of 2%.
I have also noticed that by testing individual files "FastBackup" with level from -m4 to -m9 compresses better than "ZSTD" with level from -5 to -21, and viceversa.
The two compressor will equate, and "FastBackup" also remains a valid compressor.
P.S I sent you a PM.