Well, we can use memoryview for the dict generation avoiding creation of string objects until the time for the output:

    import re, operator
    def count_words(filename):
        with open(filename, 'rb') as fp:
            data= memoryview(fp.read())
        word_counts= {}
        for match in re.finditer(br'\S+', data):
            word= data[match.start(): match.end()]
            try:
                word_counts[word]+= 1
            except KeyError:
                word_counts[word]= 1
        word_counts= sorted(word_counts.items(), key=operator.itemgetter(1), reverse=True)
        for word, count in word_counts:
            print(word.tobytes().decode(), count)

We could also use `mmap.mmap`.