In CSS we can use several different methods to define a color:
red#FF0000
I used the same tool from jsperf.com that the others did, and created my own test for different color formats. I then ran the test on IE11, Edge17, FF64 and Chrome71 and gathered all results in a compact excel spreadsheet.
Top three are green, bottom three are red, best and worst are bold.
I don't know why Chrome is so prone to named colors format, but it made me repeat the test many times with the same and different parameters. Results remain constant.
You cannot get conclusive results of any one format being the absolute best, but my conclusion is as follows.
I will keep using hex over named, lowercase over uppercase and start using short over long hex when possible.

Feel free to update results if they change with new versions of browsers.