The problem with mineral build up is how it seems to suddenly happen. Since Lavy Hot water flow is normal, that points to the calcium chunk in the cold water inlet valve of the Lavy faucet. NOT the exit strainer since the hot water is normal. You need to pull the Cold water valve body of the lavy faucet. You need to pull the water line off the input of the toilet. You will probably have to pull the toilet and look at the water valve inlet of the toilet. It probably has Mineral build up. You either pull the toilet water valve and rinse it out or just replace the toilet water valve. PRESSURE is not your problem. FLOW is. Doug