Husqvarna Lt100 ride on

I suspect you have a wiring harness problem. It also depends on which version of the wiring harness you have. I can only find one wiring diagram but not both versions as the mower has either a single or double pedal switch. If it has a double pole it is possible that seat switch connector has a shorting strip built in to prevent operators from simply unplugging the seat switch and putting jumper in is the same as the operator being off the seat.

Doc you are killing the patient. Hope your Malpactice insurance is paid up.
 

Ive joined the 2 wires which go into the switch for the seat together

 
That is the wiring problem. It is suppose to be an open circuit with the operator on the seat and not a closed circuit. And it does sounds like you have the dual pole pedal switch has on pole closing as depress the pedal to complete the starter circuit and the other pole closes as you release the pedal. As you finish releasing the pedal this closed pole feeds through the seat switch circuit to ground. At least one these switches must be open as to not ground the magneto.
 
Of course it moves very slowly when the clutch pedal is only part of the way out, the drive belt is still slipping as intended.
 
Now there are mowers that requires the seat switch to be a closed circuit as relays are used to ground the magneto and to disable the fuel anti after fire solenoid.