Surely a loop over regexes is not the smartest solution that one could use for this? We do have algorithms for looking up multiple patterns at once and such, right?

I think the fastest data structure for this is to replace the whole thing by one big finite state transducer https://blog.burntsushi.net/transducers/ - such data structure can be built from the EasyList regexes, and perform all checks in a single lookup.

The idea here would be to compile the regex of each rule to an automata (each automata matches a set of strings) and them build a fst out of them (that would map strings to matched rules)

Is it really the fastest way? I imagine that without extra data it may be, but what I was proposing in a cousin comment would involve profiling and early fast rejection by means of efficient multiple string matching. The browser should be able to collect a significant amount of historical matching data to optimize these lookups.

Well, you can see one approach in the uBlock source code.

Matching against static, glob and regex patterns. And there is some grouping and filtering so, it's not like every URL gets all 64k patterns matched. And it's possible to pre-compile and group the Regex to get a little faster (lots of framework routers do this)

From my opinion the "web" is so contaminated it's a shorter accept-list than a constantly changing reject-list

I imagine that one should be able to profile the actual data from the actual web pages and then compile an optimal (or at least a near-optimal) decision tree, i.e., a decision tree with a minimum expected value of testing any of the inputs seen so far. Worst case is that this comes out as equivalent to the naive loop (the naive loop is essentially a degenerate decision tree), but average case should be much better.

Furthermore the decision tree needn't be binary. If you group regexes by common substrings and then even group substrings for, say, a Commentz-Walter matcher, you are able to distinguish N+1 cases for N patterns, with significantly lesser costs than N times the cost of matching for one substring alone.

Personally use an "allow list" as opposed to a "block list". Not sure which is "easier" but definitely know which one is faster, more efficient and has zero misses or false positives.

Arguably its more useful to know what a given website actualy requires than to know every possible advertising scheme. Allow lists teach about where to find desired content. Block lists do not teach anything useful in that regard.