This not a script to benchmark or determine the execution speed of PHP scripts. If you're looking for one, check out the PEAR Benchmark package.
This script sends 512 KB of HTML comments to your client. Parsing that HTML may add to the total transfer time, so don't take this as your raw download speed.
Transferring 512 KB
Transferred 512 KB in 0.011 seconds, more than 512 KB/s