A Level = Hard and really in depth. Most people do 3/4 over 2 years. Allows people to play to their strengths, i.e. pick subjects they are best at. Now that it is modular, it actually isn't as difficult, given that most people are not doing anything else massively time consuming alongside them, but you still come out at the end knowing a lot about your subject, equivalent to freshman year in the US.
IB = Moderate difficulty exams, but you have to do what, 5/6 subjects? IB is perceived as difficult because there is little consensus from universities in terms of how they ought to compare it from A levels - some think 36 points is equivalent to AAA but some (no one's looking at you Oxbridge) make offers at 40 or even 42 points, which means you have to ace a lot of exams, difficult in a syllabus which doesn't allow people to play to their strengths, ie. a very mathsy person would still need to do a language etc.
AP - I have to be honest, these do not look difficult. Forgive my arrogance, but the content of the exams does not seem hugely strenuous. BUT they would become really difficult when you are expected to do 5 or 6 of them, as well as the appalling levels of extra curriculars high flying US students are pressured into doing. It would be reasonable in my (admitedly limited) view to expect a good student to nail a few APs in a year, but if they are waking up every morning for swimming practice, directing a play after school, student president as well as completing the requisites for a high school diploma (don't forget that most students do APs as an advanced extra, on top of a normal school day).
So yeah... I suspect actually that US students work harder. I have met a lot at top us colleges and they really are very balanced, hard working and competitive in a way that UK peers are not. In my view, the American system is geared towards producing innovative, flexible and creative members of society who can contribute in a multitude of ways. Britain's whole academic system is geared towards producing specialists. Even nominally open ended degrees like English or History effectively create graduates who would likely struggle when asked to use basic calculus, but would be really good at English or History in a way few US peers are. Of course the skills they pick up along the way with a focused humanities degree means they are much brighter prospects for academia, law or bureaucracy than US peers (evidenced by the much faster process of becoming a a lawyer in the UK than America - Brits are further along on the way to being a lawyer than a Yank of the same age)