There is no limitation, the mIRC scripting language isn't fast, but not that slow.
Without seeing the script it's impossible to help you, but that being said, you did mention you are using timer, and you said how using 0 as the interval fixed it. It's not a fix, it's the correct way to get the timer to fire immediately, that's how you should do it if you want it to fire immediately. If you were using the basic /.timer 1 1 msg #chan message, this is a one second delay.
