Permafrost responds slowly, then not slowly
The word permafrost describes a condition, not a material: ground that has stayed at or below zero for at least two consecutive years. Above it sits the active layer, which thaws each summer and refreezes each winter.
The interesting behaviour is at the boundary. Thawing ice-rich permafrost does not simply warm; it loses volume, because ice occupies more space than the water it becomes and because the ground structure was partly held up by that ice. The surface subsides, sometimes by metres. Subsidence collects water. Standing water absorbs more solar energy than vegetated ground and conducts heat downward more effectively. The thaw accelerates.
This is why permafrost degradation tends to look like nothing happening for a long time, and then happening quickly in a localised patch. The slow phase is the active layer deepening a few centimetres per decade. The fast phase begins when a thaw feature forms and starts to feed itself.
Predicting where the fast phase starts is hard. Ice content varies enormously over short distances, and the ice is invisible from the surface. Two sites a hundred metres apart, indistinguishable in a photograph, can behave completely differently.