I'm looking for a story I read in the mid 2000's (though I believe the book itself was older than that) in which the protagonist encounters a society in which the inhabitants can speed up and slow down their subjective experience of time. Those that slow down their time-stream appeared to everyone else to move slowly while those that sped up their time-stream appeared to speed by or become blurs. In order to have intelligible interactions with each other people are able to sync up their time-streams as they wish.
These choices each have their own tradeoffs. Going slowly through time means that you could live longer relative to other people and therefore see further into the future, but the trade-off is that you will see everyone around you live their lives and die before you do. Going quickly through time means that you can do a lot in a given timeframe, but you would die quickly compared to everyone around you.
Some people took these choices to the extreme. If I remember correctly there was at least one person who moved so slowly that he appeared to not move at all. He was seen as a kind of guru because he had seen so many people live their lives and had lived through so much history. However in order to talk to him you had to slow your time to match his, which meant that every minute you spend talking to him cost you years compared to everyone else, so after a conversation with him you might return to your normal time-stream to find that everyone you knew had died.
I don't think that the whole book was about this concept, but it was the part that stuck with me the most. I've seen numerous other story identification questions about The River of Time, but from the descriptions of it I'm quite sure that that is not the story I'm remembering. In this story the time dilation is voluntary and changeable and was not viewed as a disease.