They absolutely will for some races, not for others. Singapore, Canada and a couple of others are very heavy on fuel and would absolutely be faster with more fuel in, others would be massively slower.
I've said time and time again, a fuel limit for the season with different length races and different styles of tracks is beyond absurd. It's also not really needed with a fuel flow limit, the fuel flow limit dictates what the perfect amount of fuel is to get round in the fastest time and changes on a per track basis. If they didn't have a fuel limit they would probably use say 105-110kg Singapore/canada, 70kg Monaco... 95kg somewhere else, 100kg at a couple of tracks.
But there are races where another 5-10kg of fuel would speed things up, only a small amount and it would barely be noticeable.