So, I've taken randomly "Bravo Hits 57" compilation and created wav image files with external CUE files. It's modern mastering pop music with some acoustic tracks. Encoders are:
e:\WORK>flac -v
flac 1.4.0
e:\WORK>ffmpeg
ffmpeg version N-107264-g23fde3c7df-gc471cc7474+3 Copyright (c) 2000-2022 the FFmpeg developers
built with gcc 12.1.0 (Rev2, Built by MSYS2 project)
Writing application : Lavf59.25.100
(ffmpeg was compiled on this machine couple of months ago)
Parameters for encoding were these:
flac -8 and flac -8e
ffmpeg -compression_level 12
Results are as follows:
Various Artists - Bravo Hits 57 CD1.wav 766,37 M
Various Artists - Bravo Hits 57 CD1 (ffmpeg-l12).flac 539,54 M
Various Artists - Bravo Hits 57 CD1 (flac -8).flac 538,24 M
Various Artists - Bravo Hits 57 CD1 (flac -8e).flac 538,12 M
Various Artists - Bravo Hits 57 CD2.wav 740,79 M
Various Artists - Bravo Hits 57 CD2 (ffmpeg-l12).flac 526,02 M
Various Artists - Bravo Hits 57 CD2 (flac -8).flac 525,00 M
Various Artists - Bravo Hits 57 CD2 (flac -8e).flac 524,91 M
So, to note, I didn't measure time, but flac -8 was subjectively fastest, I think ffmpeg was a bit faster than flac -8e. ffmpeg showed encoding speed as 40x, not bad.
To conclude, there may be cases where ffmpeg flac encoder will "win", but it all depends on type of music, mastering, noise levels. To conclude it has better compression all the time over official flac is stupid.