Obviously, most accidents happen while the bike is in relatively high speed, which
Please cite your sources. This is not obvious at all, and I would tend to disagree with your assertion.
1) makes it harder to respond to unexpected situations,
This would depend entirely on your bike handling skills. You should never exceed what is safe for your level or expertise, nor what the situation dictates.
2) makes it harder to come to a stop,
While this is a simple matter of physics, stopping is not always the safe thing to do. Again the situation dictates what is safe. I would suggest that stopping on a 35+ mph multilane while you are attempting a left turn and are in lane would not be a safe choice...the examples are endless. Likewise, stopping essentially eliminates your ability to maneuver, so again it depends on your bike handling skills.
3) worsens the impact (obviously)
Also a simple matter of physics, but getting run over from behind because you chose to stop, or significantly slow down in a higher speed lane (whether from a car or another bicycle) is not my idea of lesser impact.
If I maintain a reasonably medium or low speed on the city street, it will be less risky? (Of course there are occasions when faster means safer, such as when you are avoiding a car coming right at you: in that case beling slow would mean waiting to be hit
)
There is no one size fits all. As many posters to this thread have indicated, slow is not better, it is just an option that is dictated by the situation in which you find yourself riding.
It is the same with driving a motor vehicle, speed is dictated by law and limits, but in all cases, what is safe and reasonable at the time with the conditions present.
HTH
zac