When proxy_http (and I assume the other proxy modules) make a connection to the backend, a DNS lookup is potentially performed for each request. A simple DNS cache, flushed on server graceful restart, should improve performance here. To make this configurable, a timeout should be set as to the age of the entires in the cache, 0 meaning "don't cache". A typical value might be 1 minute or so - short enough so that admins who change the IP in DNS and then don't see the change in their httpd installation won't be sent on a wild goose chase, but long enough to make a significant difference to performance.
Why implement in httpd? Why not use existing solutions e.g. nscd works well.
It would have to be investigated whether something like nscd would give enough of a performance enhancement, or in environments where this is not possible (win32?) whether a cache would improve things. This is why this bug is marked as "enhancement" :)
Windows Server has its own DNS implementation, and recursive DNS servers such as Unbound are available cross-platform, so the Windows case is covered. Of course there's no httpd code to benchmark against, but external cacheing resolvers offer good performance without adding complexity.