I don't know if this is really an answer, but I've had a Coleman 6 person spa in service for about 25 years. Yes, 25 years! I have never had the same problems you are experiencing. We learned early on that any clothing (bathing suits etc) must be very clean and rinsed extremely well before going into the spa. Clothes cleaning detergents left in the fabrics release into the pool water and cause foam quickly. The clarifier is good at flocking these soaps, but they do tend to gum up the works quickly. We never lost a lot of pressure however, just had to clean the filter. We also made a habit of completely changing the water at least twice a year. After draining as much as we could via the drain hose, we used a wet vac to get as much out of any low spots. We then put in just enough water to cover the intake close to the bottom and jogged the pump to cycle clean water through the system and get the dirty water out. Again, drain and vacuum and repeat this process until the water looks clear. We do this with the filter out. This will clean out a lot of crap. About once a year just before we do a drain and complete cleaning, we treat the tub with anti-scale cleanser. This cleans the internal pipes of built up scale and solids. In your case, you may want to do this anti-scale treatment for a few days with the circulator on low and the filter out to purge the internal system. Then go ahead and change the water.
Another test you can do is to see how well the pump pushes water. When you have the small amount in the bottom, just above the intake, jets exposed, turn the jets to full, air off and see if the pump will shoot the water out of the jets with extreme force. Mine will shot water 20 feet away!!!! lol. If you don't have really good pressure, you may have a warn pump impeller. They can be damaged over time by chlorine based sanitizer, I never use chlorine, just bromine based shock etc.
The other item to check is if you have the right filter for your unit. Filters come in so many styles and densities. You may need to find one that allows better flow.
Assuming you find the root cause of your problem, good housekeeping, keeping as much contaminates as possible out of your tub will help. Very clean bathing suits or none at all,(yahoo) no body cosmetics such as body oils, moisturizers, perfumes etc. Keep chemical treatments to a minimum. Do a quick check weekly for PH, Total alkalinity, and bromine level. Adjust them sparingly so not to overdo any chemicals. Once you get everything under control, maintaining it is very easy and quick. Good Luck
I finally went ahead and swapped out the filter cartridge. The metallic taste is gone.
So, I believe that my theory was probably correct.
I'm not going to try to flush out the filter cartridge; I'm just going to throw it out.
So, lesson learned: when replacing a filter cartridge, make sure the inside of the filter enclosure is fully clean; don't just quickly rinse it out.
Best Answer
You likely have air trapped somewhere it shouldn't.
Water doesn't compress so when you turn on the tap, the flow is the pressure from the city divided by the resistance of all the pipe between it and the tap.
If you have an air bubble, then when the tap is shut off the air will compress until the pressure matches that from the city. When you turn the tap on, the flow will be the pressure from the air bubble divided by all the pipe between it and the tap. Since there is much less pipe in this case, the flow is much higher.
The pressure in the air bubble quickly drops as the water is pushed out until you again match the original case.
This is especially noticeable if you have have a smaller pipe somewhere between the city and the air bubble than between the air bubble and the tap since obviously the flow is limited by the smallest (cross-sectional-area) pipe.
Depending on the size/shape of the filter, it could be air in the filter itself. See if you can fully fill it with water before connecting it.
If you shut off the house water while making the change, then air could have gotten into any branch going to a higher location than your sink. If you haven't tried running fresh water in every single location in your home, that may be all it takes to clear out the air.