If you power off the problem machine does it then improve on the others?
Sounds like whatever is on that machine is using all your available bandwidth, so you'd be wanting to check for anything that can do that.
Unless you have traffic shaping or QoS configured on that network segment the gateway will be likely working on a best effort algorithm - meaning if you have a torrent client, sync operation. or the unit is compromised as a DDoS zombie or something similar it will use all the bandwidth you have, starving the other devices.
No-one has just dumped 500GB of pictures into a cloud-sync enabled folder for example? Dropbox etc.? That kind of process will just keep going as fast as it can until it's done.
These processes may not use much in the way of machine resources - the dropbox client for example is very lightweight, bitcoin mining would be very high machine utilisation, but low bandwidth - the machine would feel slow, but the network wouldn't
On consumer networking equipment this is more difficult to narrow down quickly than enterprise as the features aren't there to monitor traffic in detail.
A decent way to at least prove this is a likely issue is on a non-problem pc start a command prompt and type
ping -t 8.8.8.8 and wait for the response times to come back and stabilise - likely be 7-25ms
then fire up your problem pc give it a few mins and then go back to the running ping test and see if you're starting to see higher response times or timeouts