Hi,
it's very hard to give you reasons why your customer is experiencing this quite uncommon issue. From the description given it appears the issue regards much more the Ethernet setting parameters than Standard ERP...
So I have more question to ask in the aim of having a wider idea of what is happening there:
Does problem appear only when OKing invoices or the ethernet slowness affects other actions also?
Is the server on-premise or in cloud? and how the server is connected to the network?

I would check if there is a VPN or a Proxy running, first. I could say any sort of "unusual" setting that could compromise the data routing on the network, then eventually port forwarding and the server firewall settings. What OS does the server runs?
Any HALcust running?

as many details you will give us as many possibilities we will have to figure out why and how to fix.

Best Regards
Giuseppe
HansaWorld Support