The author rightfully points out that many sites use IFRAME tags for ads, and that can slow down your site. However, the alternative is to use a Javascript ad

The author rightfully points out that many sites use IFRAME tags for ads, and that can slow down your site. However, the alternative is to use a Javascript ad tag. I've learned from painful experience that if the ad server doesn't respond to a JS ad tag, the entire page will stop loading until the ad server times out (up to 10 seconds). I'd rather have a page be 10-20% slower with IFRAME tags than use JS ad tags that are effectively a single point of failure for my site.