When I flush my toilet the tank takes forever to refill, sometimes 30 minutes or longer. I replaced the flapper in case that was the problem, it isn't. If I detach the fill tube from the overflow tube whilst the tank is filling, it fills up in seconds because the water is coming through the fill tube fast. The problem would seem to be in the overflow tube. Do I need to replace the overflow tube or can it be adjusted in some way? Maybe there is a blockage , how can I tell?

Which fill tube's yours? Back to you, Tom

Closest diagram is the one on the right (colorful)

I set the tank level by adjusting the height of the float ball.
There are two ways to do this, 1) by adjusting the screw on top of the ballcock, **or** 2) (My personal favorite) Grasp the rod just ahead of the float ball, (do not grab the ball itself) with one hand and next to the ballcock with the other.
Bend the rod up to increase the tank level and down to decrease the level. The tank level should be set at 1/2 inch under the top of the white overflow tube. Good luck, Tom