The water pressure in my bathtub faucet has been declining slowly for months, maybe even years. Now, it is at the point where it is only a trickle, for BOTH cold and hot water. I can hardly take a shower, and it takes about an hour to fill the tub at this rate. What can I do before calling a plumber and paying big bucks? The bathroom sink in the same room has excellent pressure, as does the rest of the house. I have the two-handle Hot and Cold faucet (not the single handle). Please help! I am a novice at plumbing. I’ve never even touched it since this is the first house I have owned. I have copper plumbing in the entire house.
The showerhead is fine and pipes look clean there. The pipes must be clogged somewhere before reaching the faucet. Is there anything I can buy to clean the copper pipes?

What would cause low pressure in the bathtub faucet and shower head when both the bathroom and kitchen sinks have good water pressure? And how would I fix it? The plumbing in house is just a year old.
TIA

Recently the hot and cold water stems got new washers cause they were leaking. Now the water pressure from both hot and cold, or combined is running really slow. The shower pressure is fine, it’s just the bathtub faucet that the water is slow. Anyone got ideas?
The low pressure IS IN THE TUB FAUCET ONLY and the washers were the right size.