Grass stores its energy in the leaf. Take a third off and the plant barely notices — it carries on growing and the clippings are fine enough to break down into the soil within hours.
Take half or more and you have removed most of its working surface. The plant stops growing outward and spends a fortnight rebuilding what it lost. That is why a hard cut yellows a lawn and a light one doesn’t.
Do that repeatedly and the damage compounds. Each recovery leaves behind dead stem and runner, which builds into a spongy layer between the grass and the soil. Once that layer passes about two centimetres it starts shedding water instead of letting it through, and now you have a thatch problem on top of a mowing problem.
So the lawn left three weeks and cut hard is not saving you a visit. It is buying a dethatching job in eighteen months.