my question is: once the engine has found the node(s) `//*[@xml:id = "lemma-aMSa”]`, couldn't it stop searching for nodes that match `following::*[(self::tei:entry or self::tei:re)]` once it has found 3 of them?

Correct. Well, maybe.

Or, in other words, couldn't a positional predicate like `[position() <= 3]` be used to short-circuit the preceding part of the query? I am pretty sure that Saxon, for example, does exactly that for simple positional predicates like `[1]` or `[position() <= X]`.

Christian is actually doing a very good job enabling the compiler to catch exactely these cases. Unfortunately it is far from trivial to detect each possible scenario. With your given query the optimization might be triggered if you put the first part of the expression in brackets, like:

Note: this only works, if there’s only one node with the id "lemma-aMSa” in your document, otherwise the expressions are not equivalent. If this is not the case or if the brackets don’t fix it, I’m passing on to Christian for either an explanation or a faster-than-expected fix.