Bent pin? 

Dirty port? 

Not using the proper cable (a sketchy cable or cat 5 cable and not cat 5e, will connect at 10/100 and not gigabit)

The device you’re connecting it to? 
Many things can cause it to negotiate a non-gigabit speed. 

tough to know without more details. 

I’m connecting from Port #2 directly to an iMac with no switch or router in-between. Thanks for the tip The SBG8300 is 3 months old but I’m unsure if the problem always existed without me testing for it. I checked that there are no bent pins in Port #2. Then I changed the old 12-ft CAT 5E cable for a new CAT 7 and tried another speed test. Download speed showed 91.9Mbps. Upload speed is about 5.5Mbps on all four ports.

So it drops the speed on any of the ports it connects to? Makes me suspicious of the iMac of everything else works fine

Drops the speed only on Port #2, regardless if the connected device is an iMac or a laptop. When the other three ports are connected separately to an iMac, laptop, or desktop, the download speed is 250Mbps+ for each.

 Since Port#2 is giving a slow speed which is 92Mbps, compared to the other Ports which give 250Mbps after trying on the same ethernet cable and devices so, for further support if the modem is in warranty,  contact arris.com/consumersupport