There is a big difference in the size of the crystals of the minerals which make up the rock.
The main difference between fast and slow cooling magma is in the size of the crystals of the minerals that form.
A fast-cooling magma will be very fine grained with very small crystals. This is seen in a rock such as dolerite (diorite) which commonly occurs as narrow dykes or sills. The individual minerals are not visible to the naked eye.
A slow-cooling magma allows for large crystals of separate minerals to develop. This is often seen in massive magma intrusions such as granite. Crystals of quartz and feldspar are easily seen, usually with a collection of other minerals such as biotite as well.