As the PS thread on Giant ‘Baby’ Galaxies included discussion on the long intervals required to develop some cosmic structures, I thought this report from last year relevant.
The galactic monster is an especially large example of a radio galaxy, or a galaxy with a supermassive at the center which gobbles up enormous amounts of matter before spitting it out — sending gigantic two jets of plasma moving at close to the speed of light. After traveling millions of light-years, the plasma beams slow, spreading out into plumes that emit light in the form of radio waves. In the case of Alcyoneus, its lobes are the largest ever discovered.
Astronomers just found the largest galaxy ever discovered
Source paper:
The discovery of a radio galaxy of at least 5 Mpc
Distance: 3.5 Billion Light Years
Even at relativistic speeds, the distance traversed by the lobe jets involves travel time of millions of years.
Please feel free to add any favorite “cosmic structures that take time to form” examples to this thread.