The fastest way to read input in Python

I want to read a huge text file that contains list of lists of integers. Now I'm doing the following:

G = []
with open("test.txt", 'r') as f:
    for line in f:
        G.append(list(map(int,line.split())))

However, it takes about 17 secs (via timeit). Is there any way to reduce this time? Maybe, there is a way not to use map.