For doc purposes:

My first shot is that in my machine the os.rename isn't atomic and this could be generating an overhead somehow. Looking at the code, I found decisions being made on posixmodule.c about what `rename` function should be used:

 #ifdef HAVE_RENAMEAT
     if (dir_fd_specified)
         result = renameat(src_dir_fd, src.narrow, dst_dir_fd, dst.narrow);
     else
 #endif
         result = rename(src.narrow, dst.narrow);
     Py_END_ALLOW_THREADS
 

In my machine the second `rename` is being used.