I don't think having a load of functions in a class will affect its performance at all, just having a load of vars...the functions aren't held in memory I don't think, untill they are called...could be wrong. One thing that I am pretty sure of is that there is no way to add class functions in PHP.
class functions pertain to the class that they are a part of, so having them separate is tricky...You may think about making the big functions global, and sending the object to them...If the function aren't class spefic, just make them global or make the port of an object that is a var in the class...but in case of the latter, BEWARE:
A warning: see the post in this forum about 5 back I posted called Object Oriented help! Unless somebody can show me where I screwed up, If you include the extra methods in an object in the class, the object with the method will need to be instantiated in the contructutor and will crash PHP if the constructor is called by a subclass of a subclass...This won't bite you in the ass until your project is huge and you investment is high. beware!!