You might want to run a benchmark that actually stresses bigints.
I redact this statement. Bigints are allocated on the heap and according the Valgrind's Massif, the overwhelming majority of allocations were caused by them. I compiled the master branch of Guile and it performs significantly better thanks to this commit: https://git.savannah.gnu.org/cgit/guile.git/commit/?id=00fbdfa7345765168e14438eed0b0b8c64c27ab9