3.14003 seconds (time taken for 10 mysql_connect calls when hostname was used)
0.01396 seconds (time taken for 10 mysql_connect calls when IP address was used)
As you can see in the results, hostname resolution slows down mysql_connect significantly and the performance hit seems to happen on each mysql_connect call. This slowdown only happens when connection is made to a remote machine. When MySQL and web server both ran on the same machine, there was no performance difference between hostname and ipaddress case. So if you are running MySQL on a remote machine, use IP address in the mysql_connect call. Alternatively you can add an entry in %windir%\system32\drivers\etc\hosts file to tell your system IP address of MySQL machine. System will find this entry in the hosts file and pick the IP address of MySQL box directly without going through more expensive name resolution. Change hosts file only if you have a static IP address. Name resolution performance will depend on your DNS/WINS configuration and on your network topology. So you should run the script above to find out performance impact of hostname resolution before making any changes.
Hope this helps.