Toilet bowl fills very slowly unless the shut off valve is turned off and then on again
Some fill valves turn the water down if they detect a leak, sounds like yours is malfunctioning, that or scale, rust, gravel sticking in the valve. Some can be cleaned by turning the water off removing the top of the valve and turning the water on just a little. Putting a cup on top of the valve allows a higher water pressure. If the fill doesn't improve a replacement will be needed. Not all fill valves can be cleaned.