I was in Tanzania (multiple cities) for a month and honestly I did not perceive this there, would you have any example of what happened to you there? Honest question, as I found them very respectful of my time, but perhaps it had more to do with the relation I had with them (as a tourist, as compared to actually living there).
I could, having lived there for 5 years, but the linked article and other comments here provide the sufficient documentation don't you think?. I'm not saying it is a completely ubiquitous and homogeneous trait, and it is probably more pronounced in rural areas than in cities for example. Implicit in your question is however an assertion that I object to: that it would be a sign of disrespect to have a more rubbery notion of time than the one we are used to in our clockwork societies. I don't think so at all. This notion of 'respect' may be relevant here in the west, where everyone's life is measured in time, and everyone knows it to be so. I never felt it to be a sign of disrespect when the wedding started the day after the day it was supposed to, or when our guide up Kilimanjaro told us it was maybe five or ten minutes left to the top when it was in fact two hours. It was simply not so important to them.