Actually I would expect the same amount of time to charge to full assuming you charge both batteries (smaller and larger) with the same C rate. Since both packs use same cell types and same module cooling there would be no need to reduce C for the SR+.
But same C rate for both packs of course doesn't imply the same power (kW) while charging - and still both packs would be at 100% at the same time.
But anyway - for me it is all about how much range I get in a certain amount of time before the power is reduced too much.
On the SR+ for me the sweet spot for charging is roughly between 10% and 50-60%, i.e. 20-25min charging for adding 140-160miles. Plenty fast and the next supercharger already in range.
On our X100D (2017) the highest mph rating we get while charging is 312, on the SR+ it is 460 - amazing and all because of far better milage efficency.