$ str=$(hexdump -n 300 /dev/urandom |base64)
$ time for i in {1..1000000}; do [ -z "$str" ] ; done

real    0m19,171s
user    0m19,171s
sys     0m0,000s
$ time for i in {1..1000000}; do [ ${#str} -eq 0 ] ; done

real    0m22,733s
user    0m22,732s
sys     0m0,000s