To be clear from the outset: “Adagietto” isn’t a tempo indication.
Despite it being the title of the fourth and penultimate movement of Mahler’s Symphony No. 5, it doesn’t call for an Adagietto tempo, whatever that might be. It doesn’t mean “slightly less slow than Adagio” or anything like it. No matter how many times you have heard the claim from whomever you’ve heard it (and that includes some musicians who should know better) the term refers to “an adagio”—the general term for a slow movement—“of small dimensions.” “Adagietto” means “Little Adagio.”
