If you have any kind of bounce on the forks when running learn on weight it will be wrong. Wait until the forks stop bouncing before hitting the horn button. As for slower speed that isn't going to be caused by the weight. Only an over temperature at the drive motor, dragging brake or one of the mast switches is out (in which case you won't be able to run learn on weight). Any messages on the screen or codes in the history?
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