PagingFieldCollector (sort comparator) is significantly slower with string fields, because of how its "seen on a previous page" works: it calls compareDocToValue(int doc, T t) first to check this. (its the only user of this method)
This is very slow with String, because no ordinals are used. so each document must lookup ord, then lookup bytes, then compare bytes.
I think maybe we should replace this method with an 'after' slot, and just have compareDocToAfter or something.
Otherwise we could use a hack-patch like the one i will upload (i did this just to test the performance, although tests do pass).