A problem with the reed switch, wiring failure or faulty console can cause the elliptical to display the wrong speed.
Unplug the elliptical if it has a power cord and check the position of the reed switch if displayed speed is extremely slow when you're treading at a fast pace. The reed switch won't detect the crank arm pulley magnet if the switch is too far away from the pulley. Adjust the reed switch closer to the pulley so the reed switch detects the magnet and sends the proper speed signal to the console.
If the reed switch is properly positioned close to the crank arm pulley, check wiring between the reed switch and the console. Reconnect any loose wires and repair any broken wires.
If the wires are okay, you may need to replace the console because it isn't displaying the correct speed even though it's receiving the correct speed signal from the reed switch.