The goal is to block listed IP address with less time consuming. i have a list of 400000 IPs, i need block them on nginx , i can use geo modules and defind vars like:
geo $bl {
***
}
if ($bl = 1) {
return 403;
}

also i can get developer to write module to work like Geo modules, create variable based on IP if IP is in Blacklist or not.

now i have questions.
what is fastets ?(Geo or custom modules)
how much 400000 IPs in Geo modules consume time ? (respons and request processing time)
how many IP can i add in Geo modules ?