Robert Caro has an excellent reputation as a meticulous researcher and thorough writer.

The Passage of Power is the 4th book in the (so far) 4 book series on LBJ by Robert Cato. Cato has stated there is one more book forthcoming. If you're starting "The Years of Lyndon Johnson" series with this book, you've missed the first three books in the series. They are:

The Path to Power (1982)
Means of Ascent (1990)
Master of the Senate (2002)
The Passage of Power (2012)

The image of LBJ painted in the first books is of a very driven personality in terms of ambition. LBJ surrounded himself with people he could dominate, not always the top-of-the-class overachievers but people he could manipulate and push, and he would drive them to simply superhuman levels of productivity, late nights, weekends, answering every letter from a constituent and processing every document. His level of attention to detail was simply astounding. He drove himself as hard as he did everyone who worked for him. He was cunning and calculating, and on more than a few occasions would bend rules to get his way.

I think it's important when assessing how Kennedy is portrayed in the series, to understand that you're seeing him as viewed through Johnson's contemporaries who were extensively interviewed for the book. People who spent decades working for LBJ, the insane hours and browbeatings to get ever more done, would view anyone else in politics as lackluster in comparison to The Chief, as he enjoyed being addressed by his subordinates.

Kennedy himself, of course, is the subject of many biographies, some hagiographic tributes, and some more even handed. One difficulty in assessing JFK is that he was not just a politician, but along with the glamorous Jackie, a celebrity in the modern sense. His untimely and tragic death in office has tended to increase the legendary nature of his life and presidency, making it more difficult to find objective works on those topics than other modern politicians.

With these factors in mind, then, to your original question: was Kennedy considered lazy as an officeholder? Not particularly, but he wasn't particularly outstanding, either. His 3 years in office were marked by some of the roughest periods of the Cold War: Bay of Pigs, Cuban Missile Crisis, and Berlin Crisis. We managed to avoid open war with the Eastern Bloc, and Kennedy had some commendable moments in his handling of those affairs; and some serious blunders. His domestic legislative accomplishments were modest - many of his programs would be brought to fruition quickly by LBJ after he succeeded to the Presidency. There is no doubt that LBJ was by far the more effective politician in terms of wielding power effectively in the U.S. national political arena. It's equally true that Kennedy was far more charismatic on a personal level and his popularity has endured and even increased with the passage of time.

I don't have my print copy available where I'm at at present, and there's no eBook, so I'll have to paraphrase "Coming Apart: An Informal History of America in the 60's" by William L. O'Neill from memory. O'Neill's assessment of Kennedy, writing in the early 1970's, is that few presidents we consider great today would have been judged so had they died in the 3rd year of their first term. If Kennedy could have avoided disaster in Vietnam, and dealt effectively with the contentious and divisive civil rights and economic issues, he might be been judged better than average. He faced several cold war crises and didn't start a nuclear war. His untimely death cut short his many possibilities before we could really judge them against time.

One possible reason JFK could be seen as "lazy", was that he was in a lot of pain. He had several medical problems.

He had Addisons, Hypothyroidism, severe chronic pack pain, https://en.wikipedia.org/wiki/John_F._Kennedy#Health