As a user of libcares (which is awesome for bulk DNS lookups btw) I'll add that I've only ever needed one ares_channel per process. Having one ares_channel for every CURL-handle seems a bit excessive. This is probably the main problem here, not the kernel spinlock.

Edit: Come to think about it, why isn't the CURL-handle reused? Sounds like a new CURL-handle is inited for every request, which I don't recall being necessary.