When your hair is wet, the water molecules cause it to expand, thus leaving it more susceptible to breakage and split ends. If your hair takes forever to dry, it may actually be a good sign, regardless of how annoying it may be.
Healthy hair takes longer to dry because damaged hair releases moisture more quickly as it’s very porous. Damaged hair quickly absorbs water but loses moisture just as fast because the cuticle never closes, resulting in fast-drying hair.
Makeup Muddle explains that the thicker and curlier your hair is, the more time it takes to dry because it's more porous and can absorb more water. This works similarly to how a damaged cuticle lets in more water — but when hair is highly porous, it naturally has more gaps.