I've seen this exact post a hundred times: "I pay for 500 Mbps fiber and I only get 80." Usually the ISP gets blamed, sometimes the router. Most of the time it's neither. It's one stupid link in the chain quietly running at 100 Mbps.
The 95 Mbps tell
Here's the thing to understand first: 75-95 Mbps isn't a random slow speed. It's a signature. 100BASE-TX (Fast Ethernet) is a 100 Mbit/s link. Take off Ethernet/IP/TCP overhead and the practical ceiling lands right around 90-95 Mbps. So when your speed test keeps landing in that band no matter what, you're not getting a throttled 500 Mbps plan. You're getting a 100 Mbps link, and every packet is running through it.
No amount of ISP complaining fixes that. The link is the bottleneck, and the link is yours.
Why a link falls back to 100 Mbps
Gigabit (1000BASE-T) uses all four twisted pairs inside an Ethernet cable, bidirectionally. Fast Ethernet only needs two of them (pins 1-2 and 3-6). That's the whole story in one sentence:
- A plain Cat5 cable (not Cat5e/6). Cat5 is only rated for 100 Mbps.
- A damaged or bent pair.
- A crimp or termination where only two pairs got wired. This is the big one. Lots of cheap patch cables and lazy punchdowns only carry two pairs, and they work fine at 100 Mbps, so nobody notices until they need gigabit.
- A 100 Mbps-only LAN port. Many cheap ONTs (the fiber box on your wall) have exactly one gigabit port and the rest Fast Ethernet. Plug the router into the wrong one and you're capped at the wall, not the cable.
- A marginal link that auto-negotiation quietly downgraded to 100/10, and it stays stuck until something forces a re-negotiation.
How to actually fix it
Don't start by calling the ISP. Work the ladder, in this order:
1. Read the negotiated link rate, not a speed test.
On Windows: Ctrl+Shift+Esc → Performance tab → Ethernet, look at the link speed. 100 Mbps vs 1.0 Gbps tells you everything. On the router side, the Deco app shows the WAN status. On most ONTs the port LED tells the story: amber/10-100M vs green/1G, check the sticker on your unit. If it says 100 Mbps anywhere, you found the bottleneck.
2. Swap the ONU→router cable.
Get a known-good Cat5e or Cat6 patch lead. If you're crimping your own, verify all 8 wires are terminated to T568A or T568B. All of them, not just the two pairs Fast Ethernet needs.
3. Use the gigabit port explicitly.
If your ONT has multiple LAN ports, make sure the router is in the gigabit one. Same on the router if it has a separate WAN port.
4. Isolate the sides.
Plug a PC directly into the ONT with the same cable and speed test. Getting 400+ Mbps? ONT and ISP are fine, the fault is in the router/Wi-Fi path. Still ~95? It's the cable or the ONT port.
5. Power-cycle both the ONT and the router after the cable swap.
Auto-negotiation doesn't always re-run on its own when you plug in a better cable. A stuck 100 Mbps link can stay stuck until the link drops and re-negotiates. Pull power on both, wait 30 seconds, plug back in.
6. Still capped?
Test a second known-good device to rule out a NIC pinned to 100 Mbps in the OS/driver settings. And don't force speed/duplex manually. Set everything to Auto Negotiation, or you can lock the link to 100 M for a different reason.
When it's not the cable
- The ONT's LAN port itself is 100 Mbps-only. Sticker says so, you need a different port or a different ONT.
- Your router's WAN port is 100 Mbps. (The Deco S7's three ports are gigabit, so if you're on one, that's not it.)
- A degraded switch port in between.
All of these route back to the same question: what did the link negotiate?
The one thing to remember
A speed test tells you the result. The link rate tells you the cause. Gigabit plan capped at ~95? Don't chase the ISP. Go find the 100 Mbps link and fix the thing that's making it slow.
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