My niece had mono, and it was very much like Johnny's. She had been having periods where she was active all day but then got listless and exhausted at night, with a low grade fever. She didn't exactly seem sick, but she wasn't her usual upbeat self. This went on for a couple of months, then the swollen glands on her neck surfaced, and she went to the doctor and was diagnosed with mono.
With mono, the effects of the disease become worse late in the day, when the body's stamina and endurance are at a low. As the Men's FSs at the major events were in the evening, it makes sense that Johnny fared worse at the FSs than he did at the shorter SPs. That he was able to perform as well as he did is amazing.
Mono also tends to have a depressive effect on the mind. Johnny's speaking of "blackness" descending on him at Torino before the FS hints of this.