Given that you're stating that this is a hard-science setting (not a hard-science question) then let's break this down a little to explain the interaction between force and time.
F=ma, therefore time is a consideration in the amount of force. If a person can throw a punch at normal speed BUT their localised frame of temporal reference is lower than their environment, to an outside observer the punch is faster, therefore delivers more force.
This is just an exotic way of punching faster, delivering more force.
One interesting question I get when explaining relativity to others is why the astronaut who's travelling at close to the speed of light ages so much more slowly than his twin back on earth - if relativity is based on the observer, then the speed of both men is close to the speed of light relative to each other, right?
Well the problem with this thinking is that relativity is not about relative speed, it's about relative energy. The astronaut has a much higher kinetic energy than the earth, meaning his ageing is slower.
Thing is, it's the same with your person's fist. Your person's ability to slow down time is really just a fancy way of saying that he or she can speed up his or her own matabolism to a point where he or she is moving faster. Whether you slow down time or speed your movements up, you're still introducing the same amount of extra energy. That is to say, you're hero is going to have to increase their metabolism to expend the same amount of extra energy either way.
If we take relativity into account, speeding one's movements and slowing outside time are basically the same thing. In point of fact, moving faster introduces more energy, which by the astronaut example literally does slow down time. This is because space and time are basically the same thing expressed different ways, hence our 4 dimensional concept of spacetime.
In short, if you want it to be hard science, then being able to accelerate time is a confusing distraction for most people - your hero can simply accelerate his or her own movements, which amounts to the same thing. Either way, your hero has to be able to expend massive amounts of energy in a short time which also means he or she is going to get very hungry after doing this for a bit.