Trouble with Safari?

I fear that I have some bug in Safari. When running Safari, which I do all the time, activity monitor shows very heavy use, and end very quickly on top showing red colour. And the Mac slows down. On the other hand, when I use pictures, speed is quite OK. I just wonder if I might have some bug, and hope that the community might be able to help.

The above refers to my old MacBook Air with 4GB RAM.

I am writing to you on a new MacBook Air with 8 GB RAM. Activity monitor is fairly high on this on too as you can see from the image.