Your loader has 40 years of wear and tear in the linkage that controls the spool in the travel valve. Start there. Slide underneath the tractor with a trouble light while you instruct a helper to operate the travel lever and foot pedal. Watch the spool movement as it slides in and out of the valve body. Too much play in the linkage will prevent the spool from moving all the way in and all the way out. You cannot get full speed unless the spool moves the full distance.
Use the search feature on this site and type in "banana plate" to bring up past discussions on this very issue.
Wear... It doesn't take much here... The linkage is kinda complex, and there are plenty of points for wear to start compounding. I didn't really think mine was worn when I first got it. But, yea, it was, and it made a major difference to replace parts where there was any play. That's not to say wear is the only possible cause, but that's going to be the most likely...
Please use caution when working under your machine if you have a helper moving levers up top. I say this because a man was run over by his own machine around here a while back because he was in a rush to "fix" something under the tractor. Apparently, he was trying to get some debris out of the mower deck and he had a helper sitting on the tractor. The helper accidentally moved a lever while the tractor was running and it ran over the guy laying behind the tractor. He is no longer with us today.