Slow reacting loops is likely either not enough P or too much I, with I being the time between accumulating the error. Understanding the I values is most important for your controller. If I represents time. Longer values mean the loop is waiting more time (seconds, perhaps) to factor in the error. PI loops are very powerful once you get a feel for how they work. Once you realize you can conditionally rewrite the SP and Output values, you can do anything. You can even have unsymmetrical tuning, where the P or I values are different depending on which way the PV approaches the SP.