better fps

So i've been playing fps games for about 10 months now and in that time i've play for a total of 520 hours and yet my overall kd is only around the 0.550 region. Is there any tips that you can give me that can better me in anyway. I must add that even though my kd is bad, my pc that i play on isn't the best either on a cold start up i manage about 20-25 fps but as i use it more i average between 15-20 is this another factor that can affect how i play or is it just an excuse for me for being so bad?