I’d echo this.
Given the Sweetspot range can vary 10-15% I find that at the lower end of this range ‘switching off’ isn’t overly challenging, muscular endurance isn’t overly taxed, pedal strokes feel neat-tidy-even and breathing is naturally well controlled.
At the upper end of this spectrum I’m much more conscious that it’s become fairly uncomfortable but still manageable. More focus is required to keep ‘on-top’ of things, I feel the ‘lactate’ cup starting to brim but not quite overflow and breathing is understandably getting a little messier. The sooner the interval ends the better but I’m not going to be grinding to a halt anytime soon.