FFMpeg & Mplayer

Benchmarking

time mplayer -benchmark -vo null -nosound -lavdopts threads=9 /path/to/file

Change threads=9 to fit your CPU

Encode whole directory

for FLAC_FILE in '/path/to/folder/'*.flac ; do
   MP3_FILE="${FLAC_FILE%.flac}.mp3" # delete ".flac", add ".mp3"
   echo "ffmpeg -i $FLAC_FILE -ab 320k -map_metadata 0 -id3v2_version 3 $MP3_FILE"  # print cmd
   ffmpeg -i $FLAC_FILE -ab 320k -map_metadata 0 -id3v2_version 3 $MP3_FILE
done

File info

ffprobe -hide_banner "/path/to/file.opus"