The slack in the brake lines and slow reaction to turning when you use brake to initiate a turn are both good indicators that you are not using enough brake. Try one more knot of brake each time you fly, and then one more after about an hour. As I mentioned above, you're not going to like it at first, but you will get used to it. It sounds and feels counterintuitive, but the kite will actually move faster with additional brake up to a point. That is the point you want to reach. Most experienced pilots will fly with their brakes set two to four knots beyond this point, sacrificing some speed to gain exponentially more control. The wind is what determines how the kite flies, so the more of it pushing against the sail as opposed to sliding off of it will determine speed, agility, pull and precision. You are trying to achieve a balanced compromise of these to have control of what the kite does. You can fly at the extremes, for instance lots of forward drive to gain speed, but then you sacrifice the others, each to a different degree. Where you set the line on the knots is based on what you want to achieve, but until you are comfortable with ALL of the possibilities you have not learned complete control. If you ever want to be able to fly really well in 2mph down to 0mph wind, or wind that is howling at 25mph or more, you must become familiar with all aspects of tuning. And trust me, to fly in 2mph wind and look like a pro, you will be using near maximum brake, in other words setting the angle of the sail to the wind nearly as square as possible for maximum pressure to keep it aloft.
Just keep trying to use more and more brake each time and with time you will be able to feel the difference. And choose good wind to learn in as I mentioned. Bad wind is not helpful. Once you master basic control and can tune your kite to take advantage of conditions you'll be able to handle whatever the wind throws your way assuming the kite you use can handle it too.
It's a journey that never ends -- enjoy it.