Sorry about the late reply, anyway...
You need to use a timer. An easy way is to use GM's built in timers, "alarms".
You can put that code you have in an alarm event and at the end do something like:
alarm=30 //30=1 second
You will need to start it by using it in the create event as well.
You could also make your own timer.
//put your code here
There's a bajillion ways to slow it down, these are just a few.