New Ubuntu install, and the network is really slow.

Sometimes websites load fine, and not too slowly... but usually they don't, especially on first load. Thought it might be something related to dns errors but not sure. I've turned off IP6 but it doesn't seem to have changed anything.

But I have found this, running the following:
gives some strange results (replacing x with numbers).

Using 8.8.8.8, xxx can only go up to 50 (ping times about 45ms), any higher and it times out with no reply. But isn't that number the MTU, which ought to be about 1470, not about 50?

Changing the server to 1.1.1.1 gives normal results, ie it works up to 1472 with no errors.

Not sure if that's related to the slow and intermittent network problems.

Network access is through a 4G dongle, but that works fine on a separate debian system, so I'm pretty sure it's a software config thing on the new box.

Any suggestions?

I've not encountered anything like this myself, but a web search for "diagnose slow internet ubuntu" turns up a number of articles that might help. The information you've provided is rather sketchy, but I think that's more because it's such an odd problem. Some questions occur to me, though.

When you try the pings, are their significant differences in the reported response times? Also, what is the wireless chipset in that dongle?

If you test with a wired connection, is that connection reliable?

If I encountered this, I might start with hitting speedtest.net from the different machines and comparing the results. I would also move that dongle from one machine to the other and repeat the speedtest.net test.

The chipset is a huawei E3372.

I've narrowed down the problem, but not solved it - and I can reproduce it on the Debian machine too: it only happens if I use an alternative DNS server. Pretty sure it isn't a problem with the server, since I've tested it with big public servers such as Cloudflare etc.

So I can work round it by just using the DNS provided by the ISP, but I'd rather use alternatives if possible, for various reasons.

I suspect the problem is caused because it has a double-NAT setup, that I don't think I can avoid: the device uses NAT, and so does the ISP. So my PC has a 192.168.x.x address, and the "external" IP is a 10.x.x.x one. Hopefully the fix is as simple as specifying different DNS servers for different zones. Weird it's intermittent though.

I've found this https://unix.stackexchange.com/quest...ctions-problem which might be an alternative way around, but seems pretty complicated to work through (TLDR - switch the dongle mode to access the modem component directly, rather than the network device)

Ok, scrap that last post, the result on the Debian box was just a one-off slow issue, and the Ubuntu box runs slowly even using the ISP's DNS.

Speedtest.net average results show a much slower result on the Ubuntu box even on a wired network - these are typical results:

Debian box - 12 ms ping, 36 Mbps down, 9 Mbps up
Ubuntu box - 12 ms ping, 5 Mbps down, 2 Mbps up

This is definitely more stable than on the 4G dongle, but there's still an occasional hiccup with page loads on the Ubuntu box, ie sometimes they just don't load at all for a few mins, then load kind of ok (although still slower than the debian machine - on much newer and faster hardware).