

Based on ATX standards https://xdevs.com/doc/Standards/ATX/ATX12V_Power_Supply_Design_Guide_Rev1.1.pdf,
Each 12V circuit from the PSU should deliver around 150W and definitely not more than 240VA (over current protection kick in rating). One cord runs on that single circuit so it can’t deliver more than that much power. I experienced this when I foolishly thought I could reduce cable clutter building my friends’ PC, only to realize the 3080 ran terribly and was drawing about 150W, not unlike OP, except in my case it was clear this was the issue.
All hail Canada’s notice-and-notice-and-notice-and-notice-and-notice system of copyright enforcement!