When I connect my computer to internet, then my computer get slow. But when I had used Windows 10 then it was not happening. I have seen CPU usages that were using 95-100% of CPU. It happens when I connect to the internet.
You'll have to check if the GPU is well supported.
Open Software & Updates from the Software option in the Zorin Menu, Click the hamburger-menu top right corner for that.
Open TAB extra drivers and leave it open for about 30 secs. Install an offered driver for the GPU mainly.
In your browser you could try as well to turn off the hardware acceleration for the browser. Sometimes it is no more than that.
If above steps have no results please provide extra information on the hardware you are using. CPU + speed; GPU type brand, RAM amount, speed ; Brand + type of laptop or computer you are using . All these things help for finding a result.