Thanks guys!
It is the same if I have just one single row as result of a query. E.g. a query that returns a single integer, ran in the Data Explorer panel in the admin backend it takes 9 ms, ran from the outside against the Data Explorer API via curl it takes 200 ms.
I would very much like to avoid that, I use Data Explorer because it was suggested on these forums as a nice way to get information from the DB and a well working API for that.
My query uses them for JOINs but not in the result set. The SELECT only has one column and that is a newly calculated one with a arbitrary name.