I've tested this package on servers as slow as 266mhz as well as on much faster servers. The php code that is used is very efficient at looking up IP addresses in the country database.
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.
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.
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!