[Greasemonkey] iframe content loaded before or after GM scripts?

[Vectorspace]

I'm writing a script to remove ads from a specific web page. The ads are 
always loaded in iframes and the page content itself does not use 
iframes, so I am simply removing all iframes from the page. it works - I 
never see the ads.

My question is this: is the content of the iframes loaded before or 
after the script is executed? So am I just hiding the ads, or preventing 
them from being downloaded by the browser at all?