improve rayon

[TomWambsgans]

remove nested rayon loops?

[0xAysh]

Can I take this on?

My approach would be to remove the nested Rayon usage by keeping only one level of parallelism in each affected path. For chunked mutation code, I’d generally keep the outer Rayon iterator and make the inner loop sequential with normal iterators/for loops. For helper-based cases where the inner helper already parallelizes the heavy work, I’d make the outer caller sequential instead.