But your 35.000 IP addresses will certainly resemble a tree structure - that of the DNS itself. Cache your DNS lookup responsivenesses of PTR records from the highest byte to the lowest, that will result in some gain; timeouts depend on what your OS's resolver dishes out, and you can override those with alarm calls.

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}