Please allow me to expand.
I don't know how new this machine is to you or what you have done to it since purchase. Therefore, my replies are based upon YOU not knowing what the previous owner has done with respect to maintenance.
The hydraulic cylinders in a LBH hold a tremendous amount of oil. If a past owner has failed to use the correct oil, then doing a simple oil change will only remove the oil that is in the reservoir. That is not good enough if someone put the wrong oil into this machine in the past.
Now, if you reply and tell me that you removed EACH and EVERY hydraulic cylinder on the tractor and emptied them of oil 100 percent along with the hydraulic reservoir, then that would indicate to me the correct oil is in the tractor. Anything LESS, just won't cut it and we are all spinning our wheels when it comes to figuring out what's really wrong.
You are complaining about a lack of travel movement that worsens as the machine warms up. Those things point to the oil as the possible culprit. The second likely culprit is the one that was mentioned already by Randy (99flhr) which is the infamous "banana plate". This is often overlooked because the wear on these plates doesn't look to be all that much. However, you have to understand that the oil being fed to the drive motor is controlled by a "spool" that moves less than an inch. Even a small amount of play in the linkage that controls that spool will greatly affect how much it travels or in this instance... does not travel.
The spool has a center or neutral position and reverse is on one side of center and forward is on the other. If your pedal will not push and then pull the spool 100 percent, then the ground speed will suffer dramatically. What we are trying to do here is to tackle your problem by eliminating the cheapest and easiest known problems, one at a time. We don't want you to leap to the wrong conclusion. You seem to have the drive motor in your sights. Do you realize that a new drive motor may cost you $1000.00 to replace?
Drive motors have been known to fail but failures are super rare compared to the things we are talking about. Now, it could be this. Someone else thought that the drive motor was at fault and then bought a used motor from a 400 series tractor. After installing it, there was no difference in performance and so they just sold the problem by selling the tractor.
So, please.... work with us by providing full disclosure and together, we will get your tractor working like it did when it was new.