It seems unlikely that you were able to overload the Google public dns server, and it also seems unlikely that you were saturating your connection. Thus you probably hit a rate limit. If you go recursive, and cache at least the TLD servers, distributing requests evenly across the 13 logical servers that typically manage a TLD such as .com, you’ll probably see a much higher per second throughput than through the google server, but still constrained by rate limits.

You shouldn't, but as there are multiple root servers you should round robin them instead of hitting the same one every time.