I use this on Windows. For a large hosts file (several megabytes), I find the DNS client service hangs until it reads the whole file. Further, there is also a noticeable delay in the order of a few seconds until the hostname is resolved. For smaller files, I find there is no noticeable lag. Can't offer you any numbers though.
Slower DNS responses with big hosts file is something to be expected.
The solution is to feed the data to a DNS caching resolver such as unbound or dnsmasq, as I have explained in reply to UserRights.
Answering here also to point out I originally implemented this on a 5y old netbook (Atom CPU, 1GB RAM) because I tend to have a lot of tabs open in firefox, and ABP's memory usage quickly becomes noticeable, and then comes the swapping. Running unbound+nginx scales remarquably well, without hogging too much memory at startup. (Sorry, I don't have access to the netbook right now to report real numbers).
I have a netbook, so always want to keep an eye out on performance.
I wonder if there's a GUI application which enables these hosts sources to be tweaked, searched, whitelisted etc