It does not hurt to have all the browsers installed, that way you can choose which one to launch based on what you want to accomplish with the browser that does that function better than the others.
I always have at least three of them on the windows machine. Some sites even in today's age are browser dependent so the formatting is more suited to one of them, ideally all sites should be browser independent that way the user can get the best experience. I remember Ian worked hard initially when the new PC Review was upgraded and he made sure that the forum looks and functions well irrespective of which browser was being used.
As for speed of browsers, yes the graph does say which one is faster than the other but when it comes to the speed of computer sometimes the difference is negligible. Few nanoseconds faster or slower means nothing in real world computing. We humans do not react that fast! Few seconds? Yes, we can discern that for sure.
Just my 0.02 cents