I wonder if there's a correlation here with new versions of iOS being released around the same time as new phones? Is it possible that OS optimization takes a back seat to bug fixes during the run up to release then subsequent point releases improve performance?

That's been my experience, using iPhones primarily since the 6s.

The .0 releases of iOS have been getting worse lately and are not great, but usually by .2 or .3 it's fine if not better on the older devices.

I think the annual release schedule is too much for Apple to the point that now the .0 release is effectively a public beta. Especially if there are any big changes. iOS 26 RC is laggy on my 16 Pro Max, which without a doubt can absolutely handle it, it's just a buggy mess still. I'm sure that by .2 it'll be fine.