The more you zoom in, the slower it becomes. This feature helps with precision zooming when very close to something.
Here's what you should do:
Instead of zooming in to something while you (the viewport camera) are very far from it, select that thing whether it is an object, vertice, edge, face, or whatever then press Numpad . . This will take you closer to the object, then you can zoom in from there.
Always think of the viewport camera as an actual camera. When you want to take an image of something small, you don't stand very far from it and zoom in, but you get as close to it as possible then take the image.