CityFibre gigabit; inconsistent speeds (depending on server)

Hey all,

On a 1G/1G package with an ISP that uses the CityFibre national network. I’m seeing a weird but very consistent pattern and wanted to sanity-check with other CityFibre users.

Some test servers (e.g. OVH / certain Speedtest servers) hit ~930–940 Mbps down and up.

Quite a few other Speedtest servers sit around ~800–880 Mbps down, but still give full upload.

Cloudflare’s speed test is usually lower again (often around 500–600 Mbps).

Stuff I’ve ruled out:

Not Wi-Fi – all tests are wired.

Not the router – I’ve also tested directly on the ONT with PPPoE from a PC/laptop.

Not a single device – tried multiple machines, same pattern.

Not a dodgy cable – swapped them around, no change.

Not time-of-day congestion – I’ve tested at 3am and at peak times, results are basically identical: some servers max the line, some don’t.

Support say everything looks fine on their side and that their links have plenty of headroom. They basically suspect it’s more to do with how things are running on the CityFibre side / backhaul, but as long as some tests can hit full gig it’s considered “within spec” so they’re not really taking it any further.

I actually saw a very similar pattern (different severity, but same idea: some servers full speed, some noticeably lower) with a previous ISP that used a similar CityFibre-based setup. So I don’t think this is ISP-specific, which is why I’d rather not name them.

Just wondering:

Are other CityFibre users seeing the same “some servers full tilt, others stuck ~800–850 / Cloudflare lower” behaviour?

Is this just how CityFibre/backhaul/peering tends to be, as long as something can hit full speed?

Cheers!

Edit for clarity:

I’m not expecting full gigabit to every random server on the internet all the time – I get how routing, peering, congestion and remote server load work.

What I’m seeing is a very consistent pattern that doesn’t change with time of day (same at 3am as at peak):

Some external servers (e.g. OVH / certain random Speedtest servers) reliably hit ~930–940 Mbps.

The servers my ISP recommends, including one hosted on their own kit, consistently sit around ~800–880 Mbps down with full upload.

Cloudflare is consistently even lower.

Edited by leedsFTTPmate (Sun 30-Nov-25 12:17:43)

940Mbps is about the maximum that can be got from a 1Gbps service because of the packet overhead.

Thanks, Michael - I know. I'm just confused as to why my speeds are so inconsistent between the various servers. And it doesn't appear to be a congestion problem, as this appears to be independent of time of day.

In reply to a post by leedsFTTPmate:

And it doesn't appear to be a congestion problem, as this appears to be independent of time of day.

Its more likely to be the ISP you have chosen rather than Cityfibre. Given you have many ISPs to choose from, including Sky, Vodafone, AAISP and others.

Remember your ISP connects to the internet, and then buys capacity from Cityfibre to reach all the customers they have using Cityfibre's network to the homes. The same ISP will work with many companies to connect to homes including Openreach, but they will usually have the same network connections on to the internet.

Different ISPs connect to the internet in different ways. By using different speed test servers you have identified some of those links are better than others.

In the days of dialup you could choose a different ISP for each session, so if you needed fast response from a US server you could dial into someone like Pipex, and if you wanted good connectivity to JANET academic network you might use someone like Demon, or BT. You just had to pay the monthly fee (and phone call).

Find a speedtest server that works for your ISP and gives a high result for over 90% of the time. Then use that occasionally to see if something has broken or changed. ISPs rework their networks all the time, things change all the time.

In reply to a post by jchamier:

In reply to a post by leedsFTTPmate:

And it doesn't appear to be a congestion problem, as this appears to be independent of time of day.

Its more likely to be the ISP you have chosen rather than Cityfibre. Given you have many ISPs to choose from, including Sky, Vodafone, AAISP and others.

Remember your ISP connects to the internet, and then buys capacity from Cityfibre to reach all the customers they have using Cityfibre's network to the homes. The same ISP will work with many companies to connect to homes including Openreach, but they will usually have the same network connections on to the internet.

Different ISPs connect to the internet in different ways. By using different speed test servers you have identified some of those links are better than others.

In the days of dialup you could choose a different ISP for each session, so if you needed fast response from a US server you could dial into someone like Pipex, and if you wanted good connectivity to JANET academic network you might use someone like Demon, or BT. You just had to pay the monthly fee (and phone call).

Find a speedtest server that works for your ISP and gives a high result for over 90% of the time. Then use that occasionally to see if something has broken or changed. ISPs rework their networks all the time, things change all the time.

Thanks for the reply. It doesn't really bother me - the connection is great and service is generally good. I just didn't get an answer from the ISP and would just like to understand what's going on. Your answer definitely helps with that - thanks!

No problem;

Openreach is the largest wholesale network for ISPs to connect to homes.

Cityfibre is a large wholesale network for that large list of ISPs to offer service over.

Nexfibre will likely be the third.

All of those don't actually give you internet, they connect you to the ISP.

This is why Openreach vans say on the side "connecting you to your network"

Oh yeah - thanks - I know that It's just that some ISPs piggyback of the CF National network - which essentially does the routing from my home to their cabinet in London (see https://cityfibre.com/news/cityfibre-launches-nation... While other ISPs use their own core network and then connect via the last miles of CF infrastructures to homes.
My ISP mentioned that some of their exchanges "run hot" sometimes... So basically saying that the issue is on CF's side.
I guess I'll never know - speeds are high enough - so neither CF nor my ISP will investigate. So I thought I might ask if others have any clue what could cause this pattern - which doesn't appear to be merely caused by congestion (as it is time independent).

Yeah, I think Cityfibre started as regional, each ISP had to connect to the CF network in your town to be able to offer service. The CF national network means an ISP can connect at high bandwidth (thinking multiple 40Gbit, or 100Gbit) to Cityfibre in one location, and then can offer service to ANY town with Cityfibre.

Whilst this could be "run hot" its probably rumour and speculation as I doubt you'll find anything of consequence; its too easy for CF to plan this ahead of time and charge the ISPs appropriately by traffic. They know exactly where the traffic is going and how much.

As the internet itself is variable, it is really hard (at these speeds) to see issues that might be "last mile" connectivity. That is in quotes as this could be hundreds of miles, but its a managed, monitored, looked after network rather than the open wild west of the internet.

Unlike the ISP to internet, where the connections are much more complex, lots of different routes, and various options for ISPs on how to get their customers traffic to the services they want (e.g. Netflix, XBox, social media, BBC etc).

Edited by jchamier (Sun 30-Nov-25 15:29:46)

Yeah - I guess it's just one of those things. As you say, the same ISP might work very well for some, and underperforms slightly for others Thanks for your help!

In reply to a post by leedsFTTPmate:

Yeah - I guess it's just one of those things. As you say, the same ISP might work very well for some, and underperforms slightly for others Thanks for your help!

Exactly, completely depends what that ISP was building their internet connection for. For most people they work well enough for all purposes. You may get 900 Mbps say to Microsoft's downloads, and only 400 Mbps to Playstation Network for PS5 gaming... if you're a Sony fan you might choose to change ISP in the long run