The simple answer is yes. The problem is, often there are other controls used by the manufacturer or dealer, like a morse shifter. Or, You might have a Mercury box. Some boxes have a friction adjustment screw that puts a little drag on the shifter to prevent it from "backing off'" others you have to go into the box to adjust. I'd ask to place you bought it from to suggest a remedy. The cable attachments are adjustable on both ends. You might be able to adjust the engine end or you might have to open the control box and adjust it there.
Now the bigger problem as I see it, you should do a lot better than 19 mph. On pontoons the engine mounting (angle of thrust) is critical, as is the correct prop.
Here again......the dealership should have experience with this boat and motor set-up and would be able to more quickly spot the problem. If they can't help, call the pontoon manufacturer and ask them for help.