@jmw150 I'm not sure ADHD is something most people perceive as abnormal, I think most just see it as a personality type. In general for most mental diagnosis like this I tend to find that they are really more of a personality type than a disorder. Its just when it is taken too far to an extreme, to the point it can be hard to function, it can (and should be) treated.
It is a bit like the difference between healthy bouts of sadness we all have and depression. If the natural sense of sadness we all experience, or as part of our personality to feel it, gets too much its a disorder but otherwise at reasonable quantities its just a persons personality and little more.
@jmw150 Generally ADHD, at least in my expiernce, should increase your bandwidth not decrease it (if you manage it of course). Where I find people with ADHD have trouble is where the bandwidth is narrow. In other words if your asked to focus intently on a single boring task for extended period of times (low bandwidth) it can be very difficult. However when given a dozen tasks to handle all at once someone with ADHD can juggle them quite well usually.
If you had been assigned multiple tasks when you found yourself struggling with the one you suggested you would have just put it aside and worked on a different task for a bit, then once your in a more receptive state of mind move back to the original task, perhaps doing it in short sprints rather than all at once.
I find when people with ADHD follow the above pattern they can excel at their productivity beyond that of someone without ADHD.