PureOS works best on Purism hardware. On randomly chosen hardware, there may or may not be issues e.g. hardware that requires blackbox software in order to work at all or in order to work well.
What make and model of laptop is this?
Since this problem seems to be specific to the ethernet, can you provide more detailed information regarding the ethernet hardware?
How are you measuring the latency? If this is ping then I am surprised that you can get such a high latency because normally it would timeout. 8 seconds latency is terrible.
What are you measuring the latency to? Router? First hop on ISP network? Random host on the internet?
It’s always a good idea to confirm what the ethernet negotiated to i.e. half duplex or full duplex, 1000 Mbit/s or 100 Mbit/s or 10 Mbit/s, flow control.
dmesg | grep interfacename
or look at the lights on the router/switch if appropriate.
I would check ifconfig output, preferably comparing the laptop running the original OS with the laptop running PureOS.
I suppose it would be appropriate to confirm that the original OS is running only IPv4 and that PureOS is likewise - or if it is IPv4/IPv6 dual stack, that there isn’t anything funny going on there.
You would want to confirm that there isn’t something else running that is thrashing the network.
If you make no progress then a workaround may be to use a USB-to-ethernet dongle e.g. Terrible WiFi performance with Librem13, Librem15