>
> Does anyone have a theory of why pg15 should behave so differently to pg11
> here? Better still, any suggestions for configuration that might make pg15
> behave more like pg10. I am really dreading the prospect of stepping our
> many live implementations back to pg11 :-(.
>
One major factor here appears to be JIT compilation, which is off by
default in pg11, but on by default in pg12+.
You can see at the bottom of your slowest query plan that about 233s of the
240s are JIT related.
There is good info in the docs about tuning, or turning off, JIT:
https://www.postgresql.org/docs/current/jit-decision.html