The ad platform I'm using is a little more specialized for my industry, and it's in the contract that I'm not supposed to publicize our relationship without approval, so I won't mention their name. They're small enough that you probably haven't heard of them, anyway.
The reason I changed was because RPM with Adsense had been going down for the last several months. When I switched (in April), the RPM went from $1.02 to about $1.80, which was awesome, but over time I noticed that the site traffic was going down.
At first, it was negligible, but every day it was a little lower. Comparing Aug 1-24 to Mar 1-24, I'm seeing a 26% drop in pageviews (which is about 700k pageviews). At $1.80 ROM, that translates to a potential loss of $1,260.
Or, if you prefer, I'm seeing a 19% drop when comparing Aug 1-24, 2013 to Aug 1-24, 2012 (about 600k pageviews). Comparing March 2013 to March 2012, we saw a 1.5% increase in pageviews, so a negative difference is definitely not something we're used to seeing. When comparing YTD 2013 vs. YTD 2012, the difference clearly begins at the end of April.
Regardless, though, I suspect now that the slow responder was having a negative impact on search engine placement, which would explain the almost immediate loss of pageviews. Both unique visitors and pages per visit went down, and both can be explained by ranking and slow pages (making visitors leave earlier than usual).
I emailed my rep, and his tech team agreed and tentatively removed Fastclick from the network list. They're supposed to meet with Fastclick on Monday and discuss the problem. It's too soon to tell if that made much of a difference, but I'll be keeping my eye on it.
Thanks for all of the advice!