jpch - 10:05 pm on Aug 24, 2013 (gmt 0)
There are several that do this and they will all tell you there code doesn't slow down page load. IMO they all do to some extent. Whatever you do don't put an ad from one of these networks so it loads first on your page, like in the header, or before your main content. If it hangs my experience has been the entire page will hang while they complete their "real-time bidding process".