mmap is slower that caching indeed.
    Here the problem is that mmap is used even if --myisam-use-mmap=OFF

    solved by checking the flag in ha_myisam::extra() as it is called in
    init_read_record()

per-file comments:
  storage/myisam/ha_myisam.cc
Bug#40634 table scan temporary table is 4x slower due to mmap instead instead of caching 
    do nothing for HA_EXTRA_MMAP if no opt_myisam_use_mmap