If you are too tense you will need to relax and re-contract your muscles to strike, this takes extra response time. Make your fist without "clenching" it. When you strike "touch" your target with your fist a quickly as you can. At the moment of impact, tighten your muscles and drive your fist forward by allowing your shoulder and hip to follow by rotating to that side and pivoting on the ball of your foot.
The tighter you hold your fist the more your forearm tenses up. Any tension hinders punching by slowing down you movements. Beginners are told to keep their fist tight. They are told that it is for two reasons: A) they are less likely to get their hand injured if it is struck. B) They are told that the tighter the fist the more effective the punch is and the less likely that your wrist will bend spraining your wrists. Both of those are true for Beginners. BUT, advanced martial arts often teaches students to stay relaxed and tense only on impact. This allows these advantages: A) you can move your hands faster. B) You will not tire out as quickly. C) You can use what we call "Dead Hand Strikes". These are techniques delivered with the hand either totally relaxed or slightly tensed. The transfer of power to the target is very good, BUT only if you have practiced the correct way to hit through the target (Without tensing). Dead Hand Strikes are not for beginners or even intermediate students.
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Your question Jorge touches on the Chinese martial arts school of thought concerning hand speed and is one of the reasons why many of their strikes are open handed or punches start out open handed and then the hand closed just before contact. Studies have shown this to be true also and its because you closing your hand and clenching your fist involves forearm and wrist muscles and tendons as well as those of the hand itself. Your actions are faster if those muscles and tendons are semi relaxed rather than contracted and tight like when you clench your hands, making a fist.
In Japanese and Okinawan karate a lot of emphasis is put on keeping your hands closed though and that is so that lower ranking or inexperienced students don't hurt their hands or forget to close them when punching. Later and at the higher ranks though that is not emphasized so much and in a lot of styles open handed striking is not taught till the higher ranks either for students to better avoid injury.
If you look at a fighter's hands when he has them wrapped and tapped you will see that they will naturally be clenched about 3/4 of the way. A good wrapping and tape job will do that for the fighter and while not cutting off the circulation as well as support his hand and protect bones in it from not being broken. Then he only has to close it the last fraction just before contact and this also allows him to punch with his hands, wrists, and forearms being more relaxed until just before contact so that they can punch faster.
It's never a good idea to make tight, clenched fists. It tenses up your muscles, making you slower. So I guess you could say that clenching your fists does ultimately make you react and respond slower. You need to have your hands loose and relaxed. This allows you to move your hands faster. The only time you should tighten your muscles is right when you make contact with strike. This allows you to have both speed and power in your strike without having to sacrifice one.
Do you mean keeping your fists clenched the entire time you're fighting? That's a good way to tense up and not be relaxed. The most tense you are, the faster your stamina will deplete, the slower you are, the more you'll telegraph, etc
You want to try to be loose but alert.
