How to secure many WordPress Sites?

I was wondering what the best way is to secure many wordpress sites. Assuming i have a server with 100 Domains and each Domain has a single WordPress site to it. What would be the best way to protect all of this sites?

I would rather not install a plugin like Wordfence or iTheme Security on 100 instances which will heavily impact Server performance and which would be a nightmare to maintain.

The solution can cost money but should be in a realistic price range. This is why something like Sucuri which scans your WordPress instances from there server is not a solution. It costs USD299 / YEAR and adding this up for 100 instances would mean USD29,999 / Year. No thanks to that 😉

How do you protect your Sites? What is the best way to have a good balance between performance and security?

Thank you very much in advance!