[blumentopf]

I'm not surprised at all that BIND performs poorly, look at those graphs (granted this is for authoritative servers, but says a lot about BIND's performance in general): https://www.nlnetlabs.nl/blog/2013/07/05/nsd4-performance-me...

I'd stick with Unbound. There are a lot of knobs to fiddle with in the config. Be sure to compile against libevent so that you can use the highly scalable epoll as a backend (assuming you're on Linux). Turn up all the limits for cache size etc. Disable DNSSEC validation if you don't care about spoofed records. Ask on the mailing list if you need help, Wouter and his colleagues are very nice and respond very quickly.