The USAF and the companies and suppliers that are working on the new F-35 fighter jet that is overdue and over budget right now test every component of the aircraft for operation and reliability. The aircraft is capable of some very fast speeds and since it is a military aircraft odds are at least one pilot will need to eject from the jet at some point in its operational life.
That means that the ejection seat system has to be tested at the speeds the aircraft is capable of achieving. I can only imagine looking at the video of one such test of the ejection seat at 600mph that the ride for a pilot would be rough.
The testing can’t be done with a real F-35 in flight since there aren’t that many jets and crashing one would not be good. The test was conducted using some sort of rocket sled that hurtles down what appear to be railroad tracks at 600mph. The seat is ejected and apparently, that is a mannequin inside not a real pilot. It takes no more than three seconds from when the handle is pulled until the pilot is floating under the chute.