Thanks for the timings, Anon. I wanted to see how CPython compares with Guile and friends on the bigint side of things.
No problem! Especially seeing as how these timings seem to have pointed to two bugs in our scheme implementations! I have Chicken Scheme installed as well, and while I can't test the sequence implementation easily because of the library, apparently in Chicken my new implementation is competitive with yours using the python3 interpreter.
'''
R(10^13) 50000029798618763894670256 21.992 secs
'''
If there are no genuine errors in my implementation then boy is the Python3 interpreter fast!