Low Hot water pressure

I have a well and we get our hot water from our oil fired furnace.
Both hot and cold water pressure have only ever been fair but enough to get by. We've noticed a significant drop in our hot water pressure over the last month, cold water pressure is fine. We have enough to take a shower but if you have just the hot water turned on at any given location it is extremely low compared to the cold. I changed out our water filter just to be safe and there was no improvement.

Any thoughts as to what could be causing the drop? We do have a mixing valve that was installed about 5 years ago to allow some cold water in with the hot if needed to avoid dangerously hot water. Not sure if those can go bad and reduce pressure. Even with the water filter we have a fair amount of sediment that seems to make it into the water system. About a year ago we had a guy fix something on our dishwasher and he mentioned with the sediment coming through eventually may have problems with your furnace. I wasnt there though at the time.

Any thoughts?

Start by taking the aereators out of the faucets and see if the screens are clogged up. Do the same thing with the showerhead. If sand and crud is flowing in the system, those are the first to get clogged up.

Unfortunately not the aerators.

A mixing valve can fail, but (from my limited experience) more with all hot than all cold. Now, there may be some brands that fail the other way, I don't know.

Put a hose on the drain line of the hot water heater and run some water out from there...is the pressure good? Are the shutoff valves fully opened? You can buy an inexpensive water pressure gauge that will screw onto the hose bib at the HWH drain. See what the pressureis there. By using adapters, you can usually screw that into a faucet's aereator. Try turning on the hot water and see what pressure you have.

Are your supply pipes galvanized? It may be that they are all scaled up inside - they could be the size of a pencil or less inside.

good idea on checking the pressure, especially at the faucet just after the furnace to try to isolate the problem. will check it tonight.