Check this topic, first item seems to be the way to go.
This is the Keith’s motivation:
My experience is that such conditions ARE faster than :count. I believe that, the way Bubble works, if you have a condition that is testing for that state, that state will evaluate to true as soon as the list being tested reaches the non-empty state. (That is, if you are evaluating :count, bubble must wait until the entire search or filter or whatever is done to establish the count. But if you’re just checking for non-zero list length it does not have to do that – as non-zero list length is established as soon as any item populates to the list.).
JohnMark actually tested it (see one of the last post in the topic mentioned above) and came to the same conclusion.
The topic is old and a lot has changed so the best thing to do might be to just test it yourself. Creating 10000 records should not be hard. Create a couple of records yourself, use a button with copy list and it will go pretty fast, otherwise use a backend workflow.