He went on to implement https://github.com/hypirion/c-rrb Which are just like clojures vectors but has fast insertions/deletes and merges.

I semi-ported it to c# here: https://github.com/bjoli/RrbList/tree/main/src/Collections

It is faster than clojures vectors (running on the JVM, so apples and cucumbers) in all cases, and mostly beats scala's vectors except for splitting which is crazy fast in scala's vectors).