The toilet tank used to fill within 30 seconds when we installed it new 3 years ago. In the last 2 months, it takes 2-3 minutes to fill and I can hear that it is filling very slowly. The water pressure in the rest of the house is the same as it has always been.

Sounds like a restriction in either the angle stop or the fill valve(hopefully not the in-wall piping). Turn off the angle stop, disconnect the supply line to the toilet(point it into a bucket) and turn the angle stop back on, to check the flow through the open supply line. If the flow is bad, remove the angle stop and check it for a restriction clean/replace as needed. If good flow through the supply line, inspect the toilet's fill valve and clean/replace as needed.

Some toilet fill valves have an internal filter screen in the base of the fill valve where the water line connects. I have had to remove and clean Toto filters before.

I turn the water off and remove the top off the fill valve first. Then ease the water back on to dislodge any blockages.

It's probably just a swollen diaphragm in the Fluidmaster fill valve. Just replace it.