UFOs Spotted Over Denver Too Fast To See With Naked Eye

Are invisible UFOs - at least - to the naked eye in our skies? An interesting rash of sightings occurring around 1pm on different days over Denver, CO are captured on video.  Fox31 Denver reports:  "The strangest part is they are flying too fast to see with the naked eye, but when we slowed down the video, several UFOs appear."  Are UFOs out there at all times?