It certainly would be stable. A very moderate increase in speed would allow you to make good on a substantial delay for any kind of reason.

It would not be stable as it would significantly increase the risk of accidents, road rage, or being pulled over for going too slow.

That instability is not because of the 5mph but because you're driving 5mph in a 60mph zone.

You could drive 5mph on back roads with less risk of course.

agreed. the two objectives of efficiency and robustness are not necessarily in conflict in all situations, but if you start trying to optimise a given system focusing on only one objective, the other one may degrade arbitrarily. better to define what tradeoff would be a good deal: how much efficiency would you be willing to lose to gain 1 unit of robustness, etc. then optimise both objectives taking into account your preferred exchange rate.

there's plenty examples of this kind of thing in engineering design situations. it's cheaper (i.e. more efficient usage of capital, at least in the short run) to not allocate resources for backups or allocate extra capacity in systems that isn't used 95% of the time. it's much more expensive to dig two spatially separated trenches to lay independent paths of fibre optic cable to a given building, but if you cough up the money for that inefficient redundant connection, your internet will have decreased risk of interruption by rogue backhoes. it's cheaper to not hire enough staff and get individuals in a team to over-specialise in their own areas of knowledge rather than having enough spare capacity and knowledge-sharing to be able to cover if people get sick, go on holiday or quit.

ref: https://en.wikipedia.org/wiki/Multi-objective_optimization

Right, there has to be some boundaries defined for what's in "reason".