The rules are exactly the same as for variable names: start with a letter, followed by letters or numbers or underscore, maximum 64 characters (excluding the .m extension), and must not be the same as any MATLAB reserved word.
Any timing difference between different file names (that meet the rules) are so small as to be ignorable.
There have been many a flame war fought over file naming conventions, such as "camel case" vs underscores to separate words, whether the names should start with a verb followed by an object ("sort_ledger") or the other way around ("ledger_sort"), whether data-type hints should go at the beginning or the end, and so on. People have quit high-paying jobs over such matters.
None of the conventions matter to MATLAB itself: they only matter to the people writing the code, and the people maintaining the code (usually a much harder task), and to the people paying for the code (you'd be amazed how much gets written into contract specifications.)
