Those chains need frequent adjustment as the 7400 streses them. Drivers picking up double loads wont help either,
The main lift chains need to be tight with tellies down. There should be a gap at top tellie. If its nested all the way down, with no gap, stress is put on main lift cyls. They can cause cracks at base of main lift cyl mounts.
If you get intermittant codes 64 or 67 and have slow travel, check tellies to make sure they are nested all the way down. Sometimes a main lift cyl cushion spring is broken, causing the cyl to hang up one inch. The prox at bottom wont see the tellie, causing codes and slow speed.

You were right I found the problem the chain was so loose from lack of maintenance **** second shift anyways the slack was causing bouncing on the free lift so the limit switch on the bottom would go on and off, thanks for the help Joe

There's nothing from the book that I can see where weight affects speed. This is a deep reach so you might have the nested switch option (S14) which will cause slow speed.

One of our wonderful operators ran the lift into our sorter hitting the mast switch it bent the bracket so I fixed that and code X4 went away. Then I noticed that the weight comes into play as far as limiting the speed because I grabbed a pallet that was 1700 and it showed 450 I drove it and no luck it was slow speed.. Then I tilted it just right and it showed 1650 I took off again and I got full speed...No recent messages only the X4 but that went away...it doesn't make sense to me either that the weight limits the speed..

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?