Is heartworm painful?

That's also hard to say. The traveling of the worm itself may not be painful, but the inflammation it brings about inside the body can lead to some pain and discomfort and stress. Even people when they can't breathe too long or they have issues breathing, they lead to stress and anxiety. So in that aspect it might be discomforting or painful. Yes.

Heartworm tends to cause asthma-like symptoms, so the cat can't breathe. They're not perfusing, so they're very panicked. Some of their tissues may be a little painful because they're not perfusing, the pins and needles, perhaps, but I’m not sure.

No, it's not that heartworm disease is painful per se. It just causes inflammation in the heart and lungs, making it more irritating than anything else.