I have reply from mikrotik suport:
"Currently, RouterOS DNS cache has limitation of 100 concurrent requests.
As latencies grow more and more open requests will accumulate."
Do you actually have a cache? have you measured dns request concurrency?
Ouch! a shame as a local, fast DNS cache is a must, specially for big networks if you want that "snappy" feel loading web pages...
Sounds like a wonderful feature request...
Had envision a server-less network rack and I resist!!
Guess for a "few thousand" users you can approach it in another way; capture incoming DNS requests, then forward them load balancing amongst several RBs (that can also be doing other tasks) DNS caches... you gain some sort of Load balance/failover for DNS.
Another way would be going PPC (RB1110AHx2 or RB850Gx2) or a RB450G and using a linux metarouter image with a fast/lightweight dns cache, like dnscache (djbdns). The plus is it's also a recursive resolver so it can work directly with root name servers.