If things are good when devices are off and bad when they are on then implication is that issue is some activity over the Wi-Fi e.g. a device always doing something e.g. cloud uploads or an app doing a continuous status update.
You need to take things in stages
1. Wi-Fi off and computer connected by Ethernet, do things behave?
2. Turn Wi-Fi on and leave all your Wi-Fi devices turned off - if things go bad, change security keys to ensure an old key shared with a neighbour for example is still not in use.
3. If still good, turn on one Wi-Fi device and wait for 30 minutes to see if things are stable and then do the next, and if it goes bad you know which device is then chewing the bandwidth.
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