nslookup takes a long time to respond with answers under some circumstances. Looking at a tcpdump, it seems that this is when (for example) a broadband router acting as a DNS proxy, merely forwards the query response as from the real, external DNS server as opposed to from itself. Busybox/nslookup ignores the response from the external server and repeats the request. By contrast, requests made to the busybox as a proxy itself seem to be answered almost immediately. See the attached .cap file for details
I also have this problem. In the networking/nslookup.c file, find the revhost = xmalloc_sockaddr2hostonly_noport (cur-> ai_addr); Comment it, recompile, re-use nslookup, much faster.
pretty sure you're describing C library behavior, not busybox. busybox merely uses the resolver as configured by the system (e.g. /etc/resolv.conf).