List comprehension method
From the comments it sounds like you don't really need to create a new list.  Using generators will avoid the overhead of ever building a new list.

Just use this pattern:

def filterLibrarySingle(self, filterField, filterMethod, filterValue1, filterValue2=None):
    if filterMethod == 'eq':
        for cas in self.Lib:
            if getattr(cas, filterField) == filterValue1:
                yield cas
    ...


And you can call the method such as:

for item in filterLibrarySingle('my_field', 'eq', 'George'):
    do_stuff(item)


If this doesn't work, then you may have to consider designing a more advanced structure to store your data so that your searches are not \$O(n)\$ but rather \$O(\log{}n)\$.