I began to notice that the extraction of the planetarium became much more difficult, in the nullsectors, the extraction of extractors began to deplete 3 times more
the hot spots deplete over time so you will get less yield from the same spot. Also depends on the cycle time the exractor finish their cylcle in a shorter period of time, so it is probably something you are doing or not aware of rather than an issue with PI as it isnt broken in the way you describe for me.
Some resources move faster than others. I never figured out whether that is because of the nature of those resources, or because of other players on the same planet.
When other players also extract the same resources on your planet, the resources can deplete very fast. Either move your extractor heads around, set up on another part of the planet or find another planet with fewer other players.
there may be other players with facilities on that planet extracting from the same hotspot.
Not broken. Planets are variable in how there resources deplete. But the general rule is a good planet will stay good long term unless too many people use the planet for the same resource.