This is verifiable in manufactures data sheets.
Efficiency at less than 20% and greater than 80% loads isn't great relative to in between those ends.
This is compounded by lower wattage PSUs being more limited with regard to features and benefits.
If you end up with a 650w PSU and your system idles at 80 watts for the bulk of a working day you spend long periods of time in this less efficient window.
We need to see some quality 300w to 600w designs come back onto the market.
High quality cat 5e would have done the job.
Original cables must have been faulty.