I have a large horse with very long strides and can cover huge amounts of ground quickly. When asked to barrel race, he trots around barrels and takes days. He does not like the particular sport in any way.
In my opinion, if you want a successful horse that will dedicate 100 percent, they must love the work they are being asked to do. I believe it's only fair. If your horse isn't a racer, don't force it. As for a feed suppliment to gain speed - forget it. There are no quick fixes, I promise. When adding an additional energy suppliment, she with have more energy, but you can not increase her physical ability and speed. She will more then likely stay at the same pace, only longer.
She may be telling you her disapproval over running the barrels. I think it's obvious in her change of speed versus her potential speed in my opinion. Are you also taking into consideration the sprint, the anticipation between turns, the physics of a turn and the time it takes to regain speed after a turn? Your horse may be giving it all she has - I don't know the horse, so I may be making the wrong assumptions.