They are quite often better than free, blocking unwanted content that would consume more memory and CPU time than the filter itself.

Filters take a minuscule amount of resources. E.g. Even if you had to loop over a list with thousands of entries this would be unlikely to matter, and can in fact even be optimized quite significantly with various algorithms. However, because a database is most likely used, this is not even an issue, and the resource use will in fact be truly minimal.

I can not give you an exact number, but it is massively bigger than the number of trackers / elements that any usable web page can realistically implement. Probably you can do several thousands database calls in less than a second due to indexing; and that can be further optimized by doing it in batches, bringing it up to tens of thousands, if not in the hundreds on modern PCs. It is literally not an issue.