Lag on a powerful PC (Fortnite)

What to do if I have a fairly powerful PC,but began to lag fortnight,when not looking at buildings fps 150,when for example look at the poignant keys of 30-40 fps (it is high) To a minimum as well, with 200 drops to 30-40 fps GTX 970,AMD F(tm)-8320 processor,hard drive wdc wd10ezex