Its not just a problem in the subcontintent he also struggles to dominate on 5th day wickets while doing well in unfavorable conditions.

Optimal bowling speed is a very important factor as you allude. Also is his skillset. I don't know as much about finger spinners as wrist spinners but the finger spinners in the subcontinent remind me of warne in the sense that they have have 5 variations for their turn

1. the big turner - warne would always hold a little back from his stock delivery so he could have a surprise ball
2. the stock delivery - this you wanted at least enough spin so that if it landed on leg/middle it would move towards off (i.e. do more than straighten) if you are a leggie facing a RH batsmen or an offie facing a LHer. Stuart macgill could get real fizz on the ball when pitching on off but struggled to get the ball to turn pitching on middle or leg. Its a lot more challenging facing a ball that goes across your stumps
3. the slider - a ball that looked like a stock delivery out of the hand but turned less
4. the straight one - either a toppie or a flipper for a leggie or an arm ball for an offie.
5. the wrongun/doosra

Having those 5 options as well as varying the point of release and the speed made it impossible to settle against warne and the better finger spinners in the subcontinent. You are never really in against them.

Hopefully someone can advise lyon. Even if we need to recruit a foreign offspin coach