Is omitting "sort" same as sorting by _doc?

When sorting is not specified at all, is performance going to be equivalent to having specified sorting upon the "_doc" field?

No: elasticsearch will sort by _score by default.

Many thanks!

What about when a filters-only query is used (and hence _score is always null)?

A bool query that only wraps filters is still a query, it will just generate scores that are equal to zero.

Great, thanks!