I don't mind going through a single day with many rounds, but when it's spread out it's super frustrating. It feels really disrespectful of my time. I interviewed at Apple and had like 3 onsites with different teams all in one week and got the results quite quickly. It was such a good experience compared to some companies where it's such a chore. I also think the attitude of companies during interviews really sucks.…
Part of the problem is that Manton the interview panel do feel like they're wasting their time. It is valuable to the company but for the engineers, they want to get back to coding or whatever else they are working on. It isn't fair to the person applying because they didn't pick the people on the panel, but if the one doing the interviewing doesn't hide their feeling it's obviously not great.
Also, regardless of whether or not they want to be there, it's a bad look for the company. Somebody took a day off to talk to you, it's only fair that an interviewer reciprocates