Service manual states, IF owner operator suspects performance of backhoe has declined suspect Fluid level, fluid leak, fluid type and or change of specs, clogged filter and or wrong filter type which will restrict flow of system. Really only way for you to know for sure if the PTO pump is performing as it should would be to put in an inline gauge and run the pump to check its output. If not in between specs, correct all previous issues before suspect pump as the cause of problem.
In your case, it has decreased over time. Number one cause is typically wrong fluid, low fluid level or wrong or clogged filters.
I only use LS filters, even though they cost more. I know I will not have a micron filter that may restrict and cause problems down the road.