To filter URLs, you have to parse them, check if the domain is blocked using a hash table and then search for thousands of substrings in the path and query parts of the URL. If you use a regex for that, most of the filtering will already run in native code. I guarantee you that this gives you a tough to beat baseline with almost no room for improvements.

Looking at the WebKit implementation, the authors are shipping their own regex engine for whatever reason. I doubt that it beats the battle-tested re2 engine by large margins, if at all.