imo fclones is the one to beat, https://github.com/pkolaczk/fclones#benchmarks
nice that there are actually some ok interfaces here. with fclones, i tend to generate a file of candidates then do a little review to make super certain everything is square / as expected. it's very built for intermediary files a core pattern, which is very unix, very convenient. but it did take me a little while to settle on this, and it felt like i wasn't being offered a ton of options for management out of box. looks like some real attempts to be more user friendly here.
Do you know if fclones implements anything similar to https://github.com/hpc/libcircle ?
I also wonder if nftw(3) would be faster than readdir(3).