I will give it a try, time permitting. The scheme code above is a variation of something I converted from CommonLisp (orignal code by Robert Smith) in 2013. Its speed depends on bignum multiplication speed. I imagine most serious bignum implementations use the Schönhage–Strassen algorithm for very large numbers.

Thanks for looking into this! That is interesting, I wonder if it varies a great deal based on the implementation of the BigFloat rather than the language that it is implemented in.