Have you checked in the BIOS settings to see whether you can change the level of POST? Many BIOSes let you specify several levels (min, normal, max for example) to define how rigorously the POST is done. Higher levels of POST take longer to do, so it's possible you have maximum POST set??
I can't see any such option - only an option to alter the duration of the POST which I have left at 3s
Ok then. It is worth your while popping every card (and RAM) out and back in again. Also check every connector to be sure they're fully home and clicked if they're lockable types. It could be that something isn't sitting just right and that's delaying POST.
Thank you for your interest and input and time. But I think I'm going to live with it - it seems to work OK when up and running and 23s is not too bad.