losing pressure and hot h2o

Several years ago we purchased a home that is approximately 27 years old. The water pressure is adequate, however in one of the two bathrooms, the pressure started to decrease. When filling the tub, the hot water now only comes out in a trickle and may not stay hot for very long (but the sink may still have hot water coming out. When we got the house (we have a well) we purchased a water treatment system which includes a softener, acid neutralizer and a reverse osmosis system. I believe the house is fitted with copper plumbing.

Why do I experience almost no pressure and or hot water in the hot valve of one bathroom and not the rest of the house?

Take the faucet apart and replace the washers and bib screws. Clean it up and lube it with plumbers grease........

Is it a 1 handle valve?

or

2-3 handles?

It is a 2 handle, also I forgot to mention that I have oil heat, so I have a boiler and not a hot water heater.

Thanks for your help.

Do what Plumber1 said and you should be all set.