I read an article about the new MacBook Air ssd speed yesterday and I downloaded the free application Blackmagic Disk Speed Test.
I used it on El Capitan, then, I boot on Mojave and get an error message that was saying I cannot run it there because the disks (SSD and HD) are Rean Only.
Nota: to select the disk to test, click on the small gear icon on middle top of the window.
Beware: there is no visual clue on what disk is been tested (of course, it is important when you have more than one).
Also: the first run is nearly always slower than the successive ones
And: the free disk space matters on the results (of course !)