I did most of the testing of this package with a web site running on a server that runs at 266mhz. Today, that is considered slow, but the machine is only running two moderately busy web sites. So, it's a good way to test because the performance is probably similar if not somewhat better than a new high-speed server running 300 web sites. That's generally the case with web sites running on shared servers.
Needless to say, I am quite impressed with how well this runs. My own "seat of the pants" feel noticed no performance problems. I even tested it on pages that had to load numerous images and noticed nothing out of the ordinary. If I didn't know that countrycheck.php was intervening on every file request, I would not even know that something was different. Therefore, I did not feel it was necessary to perform any actual time comparisons. It works good, that's all there is to it.
On the other hand, this particular server already has my other blocking method that uses the kernel iptables to do the blocking. So, it's not being hounded by a hundred hackers and spammers all at the same time. Because of that reason, once you install this package, you might actually get a performance "increase" due to all the bandwidth savings and lack of the server being bogged down by all the idiots trying to scan your whole website for email addresses or vulnerabilities. Although, you'll also want to get the other 299 web sites running on the same shared server using this package. Then you will all really feel the difference.
No matter what, your web site will definitely be somewhat safer and more protected by keeping out all the problem countries. They want your stuff, don't give it to them!