I once compared wav to flac (level 5 compression) on several well recorded tracks using Squeezebox3 as the decoder. I could hear a slight difference in the treble, the wav sounded slightly better. I blamed the SB3, not flac for the loss in fidelity, glad to read others share the same opinion of the cause.
At a given processor speed it takes longer to compress at level8 but you get smaller file size. Since hard drives are so cheap now, <$100/3TB there is less reason to compress. And carrying a portable player full of music is obsolete compared to a cellphone streaming Spotify or Tidal. But there are songs that will never be online so flac still useful for that. I use level 5.
I did not know that flac level 0 was uncompressed, Thanks!