Depends on the software you are using.
While browsing the web, I don't notice much of a speed difference between a 1GHz CPU and a 2GHz CPU.
Now if I'm playing the latest FPS, I'll probably notice a difference.
I can remember back to when I did notice a difference in Microsoft Word performance. I don't think I've noticed a difference since about the time I had a 500MHZ PIII.
Also, it will differ for each person as we all have difference reaction speeds.
Edit: I didn't read the title correctly the first time. Due to adware... well there will probably always be a difference noticed. Just have to keep blocking, cleaning, and being careful not to catch any.