There are three fundamental sources of UFO's
If the cause of UFOs is currently unknown, and if we don't know how many unknown explanations the category of the 'unknown' might contain, then we arguably can't say how many hypothetical explanations UFOs might have.
intelligent beings far older than ourselves
We don't know that the explanation for UFOs is "intelligent beings", let alone how old those purely hypothetical beings might be. (They may be younger, but got off to a quicker start.) UFOs might be some kind of unknown physical phenomenon and have nothing to do with intelligence. We don't really know that UFOs are technology, machines in our current conceptualization. They might be alive in their own right, organisms of some unknown sort.
Those are just science-fiction speculations and nothing more than that. But they are useful for expanding the possibility space in our own imaginations, the source from which our proposed explanations come.
World war two technology from the Germans
I'd give that one an exceedingly low likelihood. The Germans had very early ballistic missiles and jet planes, but nothing like the technology that the 'tic-tacs' display, assuming that they are indeed technology.
and last but not least , government black ops research .
Yes, that's my preferred option. I do have plentiful doubts about it though.
To add , understanding speed per second matters , for example ;
At 2500mph means that per second this craft is moving at , 0.69 miles per second. Think about that .
At 5000mph is 1.38 miles per second . And so on .
This calculation of speed per/second is a calculation that nobody considers .
My point is , is when any craft seems to come from another dimension because it "blinks out or disappears " , this observation is more about speed / second ; Than about " another dimension(s) " .
That might explain how an object can seemingly instantaneously dissappear from the field of view. But it would seem to suggest instantaneous acceleration. No known propulsion technology can achieve that. And the G-loads would be tremendous. That in turn would suggest that, assuming that the observed accelerations are real, that either a) what is observed is robotic, or at least isn't organic as we understand it (subject to being squished), or b) that somebody/something has an inertia dampening field. The latter is a science-fiction possibility that's way beyond anything contemplated by present-day physics. Very unlikely in my estimation.
Another viable possibility is that the performance of these things has been mischaracterized. Dave offered up that one earlier regarding the 'tic-tacs' and I can't dismiss it.