Remember the highest priced lamb in the market is not always the most profitable - a £60 lamb off grass alone from a breed such as easycare / Hebridean x's finished in under 12 months is still more profitable than the 70 lamb that was done in 4 months, but needed creep and its mother needed ewe nuts and feed in winter.
In general I agree with you - but it isn't always so. It depends if the faster-finishing lamb can hit the market at its peak, and perhaps even without ever having needed any meds or other interventions.
Our best top-grading lambs finish in 12-14 weeks and should hit the market at a high point. So a 42kg lamb, d/w 18-20kgs, should fetch £80+. It'll have had no meds, and eaten a little cake - perhaps 1/4 to 1/2lb/head/day in the last few weeks; they don't really get on eating cake properly until around 8 weeks old. Total costs per lamb maybe as much as £2. Yes the mum will have been getting cake to keep her milk up early in the season; she may have eaten 1/2 to 1lb/head/day for the 14 weeks. Maybe £5-10 costs on her depending on the year. We don't cake much at all pre-lambing, just enough to keep them stable.
All those weeks that the no-cake ewes and lambs are growing 'for free' on grass... you'll have costs for medications for ewe and lamb (wormers, flukicides, flystrike prevention, occasional antibiotics perhaps), shearing the ewe - and less grass to make hay, so more bought in for the winter. And the longer the lambs live the more likely they are to die
, so you've losses too.
It's always a big picture in farming