Limited download speed on a USG Flex 100

Hello,
I'm contacting you because I have a USG Flex 100 (1st generation, with SFP) at a customer's with which the download speed is limited to ~200/250Mbps (whereas the firewall is supposed to take 900mbps, and 350mbps with UTM services, unless I'm mistaken). No problem with uploads (I'm up to around 580Mbps, which is more or less in line with the Internet connection's capabilities).

I've tried a factory reset, but it doesn't make any difference. Even with the UTM services, Policy Control and ADP turned off (All USG Flex services are turned off, basically), the DL remains at ~200/250Mbps.

More surprisingly, I have two other USG Flex 100s (also 1st generation with SFP) at two other customers, with similar configurations, and there, on the other hand, I have my 360Mbps (with IDP, anti-malware and content/DNS filter enabled), and even more on one of them.

I've also noticed that when I do a speedtest, the processor on the problematic USG Flex 100 doesn't exceed 60/65% utilization, whereas on the other two that work well, the processor climbs to over 90% without any problem.

All three USG Flex 100 are on the same firmware (the latest: V5.37(ABUH.0)).

Could this be a hardware problem?

Or might someone have a lead to explore on the software side?...

Thanks in advance for your help!

David

PS: Sorry for my English, I don't speak it very well, so I use a translator to help me...