Classic 3D Performance

More geometry means more triangles per chunk, so a 3D model pack is not free the way a pure texture pack is. In practice the cost is modest, because only a subset of blocks is affected and the models stay simple - but it is a real cost on very old hardware or at very long render distances.

If you are running close to the limit, lowering render distance by one or two chunks recovers more than removing the pack would.