Although i7 doesn't 100% nail down what the processor is, they're all pretty powerful and here are just copying data to disc, and that's assuming it's not just using DMA, and for a internet speed test data isn't even being written to disc. A NAS can have just a SoC and not a mainstream CPU or a PC level Ethernet chip and than can handle 1Gb/s.
My NAS is modest, but has a WD Red HDD, and even via a couple of routers/switches a file download to PC is in the upper 900Mb/s. I'm just changing from 300 to EE 900 and if I don't see 900 speed at the PC I'll be pretty sure right away it's a router/ONT/OR issue.
With fibre, the only time I've seen a speed test drop is when Ive forgotten that Ive had a download running at the same time - but look in Task Manager and you'll see the total data passing over Ethernet and a breakdown of which task is getting what amount of data - plus the disc utilisation - which can be 100% when file copying within the PC.
If one PC can transfer at 1Gb/s but the older one is only half that then it does sound like a PC issue rather than an EE issue - so I'd try asking on a Windows forum or Reddit etc - I can't think what can slow down a transfer if nothing else is running.
If downloading a game then I have seen the CPU load rise as the Antivirus seems to get a PC load - but not enough to impact transfer speed - it would need to be nearer 100% to be a bottleneck.
I'm only going to 900 because of new customer deals - 300 was enough even for 150GB game downloads - so although it's annoying that's still a good speed if there's no fix.
There's only two fixes that seem to work when PCs misbehave - if it's hardware then unplugging from the mains for a while to completely reset all devices, otherwise it's an in-place re-install keeping all apps and settings - called something like fix problems using Windows Update in recovery settings.
