There’s a long-standing bug regarding this lag that occurs (for some mobile users) when pressing the +/- buttons later into workouts. See:
Continuous Press, Quick Intensity Adjustment [Feature Request] - #67 by SarahLaverty
…and some of the preceding posts on that thread going to Nov '21.
The lag renders that +/- feature (which would be great otherwise) largely unusable for me once I’m >60 mins into a workout. I end up with intensity rising to eg 200% after (a long pressing + and then down to eg 10% after long-pressing -. It’s either 1% up/down per quick press or enormous percentage swings up/down if attempting a longer press; nothing in-between. Hence largely unusable. 
I’m unsure why it’s not been fixed yet, but assume it’s because it only affects a subset of ‘unlucky’ TR users… 