Sandor: Just like any other music! Just like with any other music! Very often he wrote down exact metronome markings, and he played those totally differently. A very good example is the First Piano Concerto. I happened to study with him the First and Second Concerto. The metronome markings in the third movement of the First Concerto are excessively fast, but all our colleagues — the honest, good musicians — all read the markings and say, "That's what Bartók meant; let's play that way." I heard Bartók play it very differently. If you follow exactly the metronome marks in that particular one and in some of the other pieces, too, the character totally changes! In the last movement of the Opus 14, which is a slow movement, the metronome marking is incredibly fast!Q: Then why did he make this outlandish marking?Sandor: That question comes up all the time. He had a little pocket metronome. Not the one that you use or I use, but one with a little string and a weight hanging on it. It wasn't accurate at all! So his metronome markings should be considered as relative markings. When 64 is followed by 80, then you know that this section is faster. But certainly do not take the absolute measurements with the markings.
Sandor was a pupil of Bartok and had this to say about Bartok's approach to such things:The full interview is quite fascinating and can be found here:https://www.bruceduffie.com/sandor2.html