//var -g is the same as /set, it just let you set more than one global variable in one line.
Yes, global variable are stored in the HDD, so yes, it's slower than using hash table for exemple.
It's just a matter of speed, if you're making something related to irc, then speed isn't very necessary, and global variable can be used, but if you're making a picture window game, you'll see the difference between using hash table and global variable.In your case, if the script is triggered constantly, use hash table, it will be faster, but you will probably not see the difference.
