OK, you say the tank fills but takes a long time. Assuming there are no leaks, the tank will hold the water it fills with. Your only issue is how long it takes to fill.
Pressure is one thing. Flow is another. You can have 100 pounds of pressure but if you're filling your tank through a pin hole it will take forever to fill. On a different hand, you might have only 10 pounds of pressure but if you're filling it through a 2 inch hole, it will fill fairly quickly.
Assuming there are no problems with the tank and its ability to hold water, your issue sounds more like a flow problem than a pressure problem. And I've seen this before. A neighbor had a slow filling toilet so she bought a new tank valve assembly and installed it. But it didn't solve her problem. When she asked me if I knew anything about it I realized that her issue was the same as what yours sounds to be. So I had her turn the water off and I disassembled the water valve on the wall. When I got it opened I could see there was a ton of calcification in the valve, choking off the supply. We went back to the hardware store and bought a new valve and installed that. Turned the water on and the toilet was full in around 30 seconds. It's been a long time so I don't remember if that was ACTUALLY the time it took, but it did fill at a normal speed, and her problem was solved.
I'd suggest you turn off the water to the toilet and drain your tank. Disconnect the feed hose from the toilet inlet and get a bucket. Put the hose into the bucket AND HOLD IT THERE with one hand and with the other, turn the water back on and see if you get a lot of water fast, or if it's a mere trickle. If it's a trickle then you've confirmed the water flow is not there.
Next, shut the main water supply off and bleed off all the pressure from both hot and cold lines. Then turn the valve on the water heater off too. Why? Because it can cause a residual pressure making water flow and you can have a spill. No sense in getting water damage in the house.
With the main off, remove the handle from the toilet valve (if you can) then remove the cover nut that holds the valve together. Remove the guts and inspect. If you see a bunch of hard stuff clogging the valve then you know you need a new valve. Don't attempt to clean the valve and reuse it. Chances are that when you need to shut it off in the future you won't be able to. A new valve is cheap enough and is good insurance that the job is done right and can be used at a later time.
If none of this helps then you might have a clogging pipe. There can be considerable buildup inside galvanized plumbing. The only solution for that is to replace the plumbing. If you have copper, they shouldn't build up mineral deposits the same way galvanized does. But copper is softer, and it's possible - though remote - someone may have accidentally hit a pipe and caused it to pinch or kink. But I really doubt that.
Or you may have PEX tubing (for all intents and purposes, plastic). That stuff is really hard to damage or kink. But I suppose if one tries hard enough, the line COULD be compromised. But I still suspect that somewhere you have galvanized pipes that have significant mineral deposits.
My home is completely redone in PEX. I love the stuff. Easy to work with and fast. Re-plumbed the whole house in about six hours time. BUT I still have a galvanized line from the meter to the house. At some point I will probably have to dig that out and replace it with either copper or PEX. Too bad there's no easy way to pull one out while pulling the other in. I'll have to dig up the whole yard for that one. Lets hope I never have to.
Best of luck with your project.