I noticed that pages on my site would hang for a WHILE, so I did some investigation.
Since breaking my site is unacceptable, Adsterra has been kicked out of the rotation.
That extremely long load time might also explain the terrible fill rate.
Bidvertiser served 3 times as many impressions on the same site over the same time period and did not prevent my site from loading.
I also noticed some weird behavior and could not easily figure out which script was causing it. Once in a while when I would click in a text box, a new tab would open to product.directredirection.com with a spammy malware site that was trying to trick me into installing a Chrome extension.
More investigation revealed that Adsterra has been known to serve malvertising in the past:
Now it’s down to Bidvertiser, Galaksion, and RevenueHits.