The C version uses OpenMP, while the Rust version doesn't. I tried running the C code with 2 and 4 threads, wall-time and CPU time don't change much in either

The C version uses OpenMP, while the Rust version doesn't. I tried running the C code with 2 and 4 threads, wall-time and CPU time don't change much in either case which is strange (this is cygwin with gcc 10.2.0): 2 threads: $ /usr/bin/gcc -pipe -Wall -O3 -fomit-frame-pointer -march=ivybridge -fopenmp fasta.c -o fasta.gcc-2.gcc_run && time ./fasta.gcc-2.gcc_run 25000000 | md5sum fd55b9e8011c781131046b6dd87511e1 *- real 0m0.724s user 0m1.468s sys 0m0.108s 4 threads: $ /usr/bin/gcc -pipe -Wall -O3 -fomit-frame-pointer -march=ivybridge -fopenmp fasta.c -o fasta.gcc-2.gcc_run && time ./fasta.gcc-2.gcc_run 25000000 | md5sum fd55b9e8011c781131046b6dd87511e1 *- real 0m0.670s user 0m1.514s sys 0m0.046s