Well I have a foil on my boat and I beg to differ. The boat does just about everything better with absolutely no adverse effects (including crossing bars and handling large following seas).
They only slow the boat down and increase fuel use if you are doing near full throttle in calm water. In real offshore useage I find I am able to go faster because the boat has a better attitude and I also use less fuel. I find I am not constantly dropping off the plane when I am forced to slow down due to sea conditions.
If a foil causes the problems HarryO describes then the boat obviously doesn't need a foil. They make a positive difference if the boat is stern heavy or if the boat is at all underpowered.