Brand new Macbook Pro (early 2015) with upgraded ram lagging in the simplest tasks

I received a brand new 13 inch macbook pro (early 2015) with upgraded ram (16 gb) about two weeks ago. I was expecting it to do tasks faster than its predecessors (the macbook pro which came directly before it, which my father owns, and the early 2009 which is now a family computer), but became dissapointed when it began crashing basically as soon as i started using final cut pro, and ever since it has been lagging everywhere else! I have check my activity monitor regulary due to this issue and have noticed "kernel_task" taking a large percentage of CPU (when im not using the computer all that much around 10%) and it uses 1.5gb of ram upwards constantly without fail! I am aware that this is the operating system so i dont really want to fiddle around with it but it does sound a little bit high to me. Originally it was slightly tougher work on final cut that would crash it, hence why i haven't seriously questioned it until now, but currently i cannot even move my curser around without getting the spinning beach ball of death! Please help me!