What sort of storage device are you using? A hard drive or an SSD? If you're using a hard drive, when was the last time you defragmented it?
What you're experiencing is one reason why I started using an iMac over three years ago. No performance annoyances or degradation at all over those three years, no malware, and no need for security software. I use an i5 (4 core, 3.4GHz) with a 1TB HDD and 12GB of DRAM and it's still pristine. I never have to defrag the HDD either. Many here bash Apple products, and Apple's prices are still a bit breathtaking, but I've stopped looking back and thinking I could get such superior hardware for half the price if I just ran Windows. My 8.1 notebook I keep around for automotive diagnostics and firmware still pisses me off every time I boot it, with updates and virus scanning.
If you don't have an SSD, I strongly recommend one for Windows use. I have a Win7 notebook at the office with an SSD, and while I can see the nasty stuff going on with the performance monitor it seldom affects my work.