Based on what you’ve described, I believe (as you have mentioned) that the most likely cause is a fault or instability in the Ethernet pass-through circuitry built into the iMac’s power adapter. The behavior you’re seeing (speeds falling from ~940 Mbps to ~93 Mbps) is typical of a connection renegotiating from Gigabit to Fast Ethernet, often due to signal integrity issues, marginal wiring inside the adapter, or an intermittent grounding or shielding problem.
Because your speeds return to normal when using Wi-Fi or a direct Ethernet adapter, this strongly suggests that the iMac itself and your router are functioning properly. The common factor in the degraded performance is the power supply block’s Ethernet path, which unfortunately can be more sensitive to internal faults than a dedicated network port or external adapter.
If you wish to further troubleshoot this issue, or at least, confirm your findings, I recommend the following steps.
- First, check the Ethernet link speed when the slowdown occurs by going to System Settings > Network > Ethernet > Details and confirming whether it shows 1000baseT or 100baseTX. If it drops to 100baseTX during the slowdown, that confirms a physical-layer negotiation issue rather than a software problem.
- Next, try reseating both ends of the Ethernet cable connected to the power adapter and power-cycling the iMac for at least 30 seconds. If the issue returns, test with the same cable and router but continue using the USB-C/Thunderbolt Ethernet adapter that gives you full speed — this comparison is important evidence that isolates the fault to the power adapter. If the speed degradation continues when using the power block but never occurs with the external adapter, the power supply block is very likely defective.
Since your iMac is only seven months old, this should be fully covered under Apple’s standard warranty. I recommend contacting Apple Support and requesting a replacement power adapter with Ethernet, specifically noting that the adapter is intermittently negotiating at Fast Ethernet speeds.
