Do you have a RAID setup? Is the chipset mode set as RAID in the BIOS?
Thanks for response. My initial reason for believing IRST was at fault no longer stands up to scrutiny. The method I used was to install the HP Softpaqs one at a time and check for the delay after each, and it appeared after installing IRST, but now it is there permanently even after a further clean install of Windows. The BIOS in this machine is so simple there are no settings for SATA control whatsoever,,,, So I am no closer to determining the culprit, I thought IRST fitted the bill as it would affect the POST, guess I shall have to look elsewhere...
Saga continues. with the Drive removed normal access to Setup was resumed, so next step was to ensure that there was nothing lurking on a drive that would cause the problem, so I cleaned an SSD with Diskpart and reloaded Windows. Same problem. Out of curiosity I tried Hibernate instead of shutting down, that worked perfectly. Not only that, but the fault disappeared! So I proceeded to load some drivers, all fine until I installed IRST. Fault re-appeared, and was corrected by choosing Hibernate just once. Puzzled...