I have been a Shito Ryu karate student for seven years now. I would like to know if there is a way to make my defences faster than they are now.
Defense is harder than offense because reaction is slower than action. You have to see and recognize the movement, your brain has to process a response, and then you have to respond. So, there is a limit to how fast your responses can get compared to attacks, even with improvement.
You can improve your speed by seeing enough attacks come in from the same style or person - you learn to read them better. (of course, this doesn't help as much when it's a different person or style with a very different set of tells for the movement).
Most styles of striking operate by throwing the force through their hips and shoulders, so you find a lot of advice involves keeping one's eyes on their chest or using peripheral vision to catch full body movement.
Some martial arts focus on getting a hand on their opponent to read their body through contact. Reaction time to touch is about half that of sight, so there's value in it, of course, you have to be in range to touch and your opponent might take advantage of that.
Some styles set up their combative stances in such a way to lure in certain types of attacks - if you know you're probably only going to have to deal with attacks in specific zones, you don't have to train as many responses - less cognitive load, quicker response. The major drawback to this is that it doesn't work if the attacker is going to grapple or if they have a weapon and you don't... in both cases, "everything is a target".
As you can see, much of the improvement to defense is sensory and cognitive, rather than simply physical reaction. However, it still suffers the limitation of slower reaction vs. action so there are upper limits no matter what.
One way is to slow down. Relax. That is to say: don't clutter the mind. When you are rigid and nervous, the whole body wants to move as one unit. This is a beginner's problem. Do you want a 150 lb body to move as a single, solitary unit - like a rock? That takes a lot of energy and muscles fighting each other. Ever see a fighter stutter-step or raise the shoulders? This is a classic tell of an inexperienced fighter.
When you relax, you allow the body to move to a more advantageous position, thereby having to move less mass at one time. Bruce Lee said it best: Be like water. He didn't say like an ice cube.
Another way is getting experience. Think of it like chess: there can be dozens of movements in the beginning of a game. Which move to take can be daunting, but, by using processes of elimination, you can limit your best choices to a handful, then select the best one from there. At the end of a game, your choices are much more limited, since there are less pieces on the board. That's how grandmasters do it, and to get to that point takes a great amount of experience.
Another way is to lead the opponent, and hope the opponent goes for the bait, and then react with the opponent. We call this "inviting an attack", and an attacker who is less experienced is more prone to baiting because it's a gross motor movement to an innate instinct. A more experienced attacker might see it also as a more advantageous opportunity and might take the bait, but better fighers are keen to look a gift horse in the mouth. Leading the opponent is a great way to limit your choices, and is directly related to your experience. Don't offer bait if you can't defend against the bait!
Experience gained is what builds confidence. When you are confident of your position and your options, your body and mind can move together without fighting itself through conflicts and inefficient responses. When you lack confidence, you gain hesitation. It is here that your brain sees the options, but because of the bottleneck in the body, is powerless to move it.