Google will only display ads if they have inventory that targets your site's content. The strange thing is that sometimes, even if they don't have inventory, they don't even display Public Service Ads like they should. I have emailed them countless times about their stupid adsense spider, and I had a reply from them saying they would look at my individual case (
www.9500communicator.com) then for an hour, all three blocks on the site had good adsense. Now two are permanently blank and I KNOW they have inventory. My problem is with the use of a Content Management System (vBulletin CMPS) in which all the portal text is actually databased in mySQL so G can't actually see it -- that's my take on it.
I do have a hack that converts php session id's into html files which Google likes, but how to hack the front portal page to get Google to behave -- no idea. I mean, it's not like their stupid spiders never visit -- they're all over it like a rash, taking my bandwidth and resources, but not applying any real logic. Crazy.
AdSense will slow down your page load -- it has to decide what (if anything) to serve you.
Good luck!