I was very surprised to this situation, but Flac Level 5 (Default mode) results gave quick results in all my tests. I've tried this hundreds of times. You can try it with any converter and see it. I think much more improvements have been made on this mode.

> Under no circumstances should FLAC level 5 decode be faster than FLAC level 0 decode.

I don't know about FLAC, but from my knowledge of compression, this result seems sensible to me.

Smaller file = less bits to process = faster. FLAC level 5 is expected to give a smaller file than level 0, so it makes sense that decoding it will be faster. Of course, it's possible that some codecs enable more codec features at higher compression levels, which makes decoding slower, so it's not always a given, but higher compression giving faster decode doesn't seem unreasonable.