Why does my Sportcraft Tx 4.9 run slowly?

I don't know about your specific treadmill, but I can offer some general advice that would probably apply to your machine. The first thing to rule out would be the tension of the belt. Unless your owner's manual gives alternative advice, to adjust the tension of the belt would require removing the plastic shell that covers the motor and power circuits located where the mains cable enters the machine. Disconnect the mains power first, then locate and remove the screws that hold this motor cover in place. Remove the cover to expose the motor and you will probably find a long threaded bolt that when adjusted will pull back the front roller that is driven by the motor. As the roller is pulled back the main belt will become tighter and prevent it slipping. A slipping belt would give you the problem that you describe.
If this does not help, let us know and we can suggest the next steps to take.

Rupert
January 2020