I'm fairly good at saying no. I would consider myself unusually productive, but not unusually busy...because I'm fairly good at saying no.
At work if a boss asks me to do something that seems interesting but where I don't have time, I'll say something like "I currently have too much on my plate to handle X. These are my priorities, should I drop something to do X?" For example, I recently used this when I was asked to do some market research but didn't have time.
If they ask me to do something I'm not interested, I'll say "I'm not very good at X, but (person B) is. I'd be happy to take something off their plate if they can handle X." I recently used this when asked to do several programming tasks I wasn't particularly interested in or good at.
If the boss insists on me doing X that I'm not interested in, or on getting everything done, I'll say something like "I can handle it this time as a special case, but we need to work out a different solution going forward."
If a friend asks me I'm much more blunt-I'll explicitly say that I'm not really interested. E.g. one of my friends recently asked me to help organize an event, and I told them I really dislike logistics work.
Overall in all of these situations I felt slightly guilty. I like being helpful and would prefer to contribute.
I receive a lot of requests, so I default to rejecting responsibilities. I generally accept them only if they seem particularly interesting or valuable.
Less empathetic/compassionate than average
Yes. Mostly I just learned political skills so that I could say no politely.
Some people seem to be a bit too generous for their own good. I know a precious few people who are especially good at saying "no" when asked to take on new responsibilities that would put them over their limits. I love working with people like that because I can always trust them to tell me when it would be better for me to find someone else to do the thing. I expect this to be an extremely valuable skill it would probably be good for many of us to understand, learn, and be able to teach to people who really need it.