Adding detail to other answers :
There are several significant differences between a skilled fighter that bullet time alone can not compensate for.
Starting with conditioning : strength, resilience, and endurance.
Strength
You might not know it until the first time someone tries to knock this three foot long stick out of your hand, but there is a significant amount of wrist and arm strength required.
Also, moving quickly requires a lot of core and lower body strength. For a concept of how much strength is required, think about potential and kinetic energy $ E = m g h = {1 \over 2} m v^2 \rightarrow 2 g h = v^2 = 2 s a \rightarrow g h = s a$: if you can jump 1 meter high (h), on $1 \over 2$ meters of legs(s), that's 2 gee's of acceleration (or about twice your body weight in lower leg strength). The average 2.5 centimeter-of-air jumper is 40x physically slower.
Resilience
When you take your first hit to sword or shield your nerves are likely to go numb and your muscles go slack. Shooters have a similar problem with recoil. Swords and shields, like firearms, transfer the blunt force trauma of a parry into the body through the wrist and arm. It does not take a very heavy hit to create this kind of injury. A fighter who practices regularly learns how to take a hit to his/her sword or shield, how to intercept an incoming hit properly, where to take the hit, and when to roll with it.
Endurance
Finally, like sports or most other kinds of physical activity endurance is essential. An unconditioned fighter's strength will likely fade in only a few seconds of full engagement. He or she will get winded, slow (because that requires a lot of strength), weak, and less accurate. A toned fighter will be able to rip your guard down and hit you.
Skills
Beyond the basics are muscle memory : accurately hitting where you intend to strike, being able to perform a parry or dodge by reflex, how to try to hit a sword at the tip, so that you get the maximum advantage of lever arm against your opponents wrists, how you engage shields near the top or side, so that you can try to curve around the shield and get in your stroke. How to lunge, dodge, cut.
The difference between a conditioned fighter and an untrained one who can see the world move in slow-motion is that many of these behaviors aren't easily intuitive. A really bright person may guess at a few, or hit a few by blind luck. Someone who studies with a good teacher is doing most of these things most of the time.
Limits of Performance, Performance Envelopes, and Tactics
Further distinguishing a trained fighter from an untrained one is recognition of the performance envelopes around themselves, and being able to estimate them accurately in an opponent. It takes a certain amount of time for swords and shields to move from one location to another, and because of that, a "window" in time is usually left open in an opponent's defense.
This is what high, mid, and low guards are. A student learns to be aware of where he or she is holding the sword, and what location of vulnerability is created by holding it there. A student also learns where to strikes effectively when an opponent has exposed an opening (and that an opening is always exposed). How they change for sword, shield or spear. Learning to perform with your eyes and body language to feint - tricking the opponent into changing their guard to accept an incoming stroke that you don't really intend to deliver (so that they are now exposed low or high for a hit).
Strategy
Someone who trains and studies regularly as a sword fighter has strategies for wearing down the opponents endurance, testing the opponent's capabilities, making use of your extended reach to keep the opponent away (if you are bigger), getting inside your opponents guard where it's difficult to wind around and hit you (if your serious), distraction, blinding, breaking weapons (when it's possible), entangling weapons or your opponent, what parts of armor can take a hit and how to work that knowledge into your defense.
Miscellaneous Knowledge
Finally, there is miscellaneous knowledge : knowing that strikes (sword, shield, fist, kick) work at a distance; however up-close wrestling takedowns are the most effective way to win. Being mindful that rocks, spears, crossbow bolts and bullets (maybe) exist, and having already given some thought into how to stay safe. Knowing when to engage, when to disengage, how to conserve your strength for when you need it.