the "speed" of DNS servers is really irrelavent as the size of a single DNS query is usually in the region of a few KB.
the biggest issue with DNS is you need the servers with the lowest latency - ie shortest distance between you and the dns server - which in zim means isp dns servers
next after latency comes reliability and quality (i'll get to that in a second) - the internet as we know it fundamentally requires Reliable DNS resolution to work, so a reliable server is needed (this means ultimately Google DNS or OpenDNS or similar) reliability can be affected by several factors:
Power, Connectivity, Overloading(this also causes speed issues)
Quality - some ISP's will cache DNS records longer than they are supposed to. every DNS record has a TTL record which means the Time To Live - or the time until the record expires and the DNS server has to go and look it up again - obviously ISP's want to make less lookups to other servers as it "speeds" things up as the expense that you might get old DNS records for some time after a DNS record might have been changed (+ its TTL)
as for Zim, i'd recommend having a decent router that runs its own DNS caching resolver, and connecting to google or opendns - you might suffer initially with the 200ms pings to Google for the first queries in a domain, but then your router will start caching the common queries and speed up dns resolution time to 1-2ms or less for sites like facebook.com perhaps
