So if you open Activity Monitor or iStat Menus and then start closing tabs in Safari, you’ll see the CPU and memory resources devoted to Safari quickly decreasing. Some sites use more resources than others, and having a lot of open tabs can slow down Safari performance by as much as 90%. If you have a gazillion tabs open, it’s really no surprise that Safari starts to struggle. This could be rather normal or hint at a deeper problem. Depending on your machine, your might experience Safari performing slower from time to time.