Same issue with my 2016 Macbook Pro 15". It was a dual monitor setup, one being a super ultra wide and one regular. No issues whatsoever until all of the sudden it was slower than molasses. Turns out my MacBook was extremely hot (In clamshell mode) even though I'm running fans at full blasts and CPU only reads 50-62 degrees Celsius. Opened the laptop for about minute and it cooled down immediately and kernal usage dropped like a brick on the CPU. My system CPU usage went from 85% to 5% after the laptop cooled down while being open. I read one response on here regarding the charging thunderbolt cable being on the left side, so I've switched to the right to check on how it would handle clamshell mode now. For now it seems fine, though I'm sure that may change here in a couple hours. Respond here and I'll try to report back with an update.