iMac 27" 2013 delay in loading boot screen after powering on

Hi all,

I have an issue with a 27" iMac, late 2013 model. When I press the power button, the system sound chimes and I see the LCD backlight turn on, however the actual mac OS boot screen or logo doesn't show until 2 minutes later after powering on the system.

When it does get to the mac OS load screen, it doesn't take long to load in to the operating system - maybe 20 seconds. The iMac specs are:

Late 2013

i7

8GB DDR3

1TB Fusion drive

I've searched internet and tried so many options. Clearing SMC, PRAM, taking out 1 RAM module at a time to see if it makes a difference. So far I'm beginning to believe the actual motherboard, or logic board, may be the culprit as I read somewhere in a forum that faulty IO ports can cause this. Is this true?

Any help on this topic would be greatly appreciated.