saving a dictionary of lists as a pickle reads the lists back as strings

I have a dictionary that looks like this for example: obj = {'a':['d','f','g'], 'x':['y','z','w']}

I save it to a pickle file using:

with open(str(file_path), 'wb') as f:
    pickle.dump(obj, f, pickle.HIGHEST_PROTOCOL)

and read it back with:

with open(str(file_path), 'rb') as f:
    return pickle.load(f)

but now the lists are strings that looks like lists: {'a':"['d','f','g']", 'x':"['y','z','w']"}

Is there a way of reading it back straight as a list?

Note: I can convert it back from strings to lists with ast.literal_eval(list) but it's super slow.

Im using python 3.8, windows 10