hi people!
i have a recurrent doubt and maybe you can help me. i'm working on mozart's k. 332 third movement and brahms' rhapsody 2, and i keep wondering about what is the minimum acceptable speed to play those pieces. 
btw, how do you define the speed to play your pieces? is it necessary to play as fast as some recordings? this particular movement, as far as i know, is played around MM=95, and it sounds very cool. but it is absolutely necessary? at this moment, i play it MM=70 and it is fine to me.
What do you think?
the same goes to brahms', and to other works as debussy's doctor g ad p.
any comment is of great help. thank you all! 
In my opinion speed is rather individual.
Certain pieces have the metronome speed printed in the page.
Some of these speed are based on pianos that were lighter and had a different responsiveness (i.e. Czerny) and hence the speed written in the sheet is just unbelievably fast and feel free to change it.
Other times there's no metronome speed but information about the tempo of the piece (Allegro, Presto, Grave, Largo ...) here you have more freedom to vary the speed as long as you focus on respecting the nature of the tempo of the piece.
But nonetheless if you have a piece which is Andante you can make (for instance) and Allegretto out of it. In other words you choose to vary the nature of the piece because you like it better that way. This is what for example orchestra directors do.
Often they have to "guess" the best speed because there's no information about it in the score, other times they just choose to arrange the piece slower or faster.
For example the Nutcracker of Giergev is unbelievably faster than common Nutcrackers recording. Giergev claims that he "sees" the Nutcracker in that way and that he prefers it faster because to him it express better the fantasy world and the adventure feel.
Sometime the best speed is just implied in the piece.
For example a piece like Schubert Impromptu op. 90 n.3 which is in 2/2 and has rain dropping arpeggios in the right hand that should sound like background to the main melodic line, would never function at a slower speed as it would destroy the structure of the piece itself.