We can all probably agree that Tenet was a pretty good movie overall. But when looked at from a scientific perspective, its most fundamental aspects fail to hold.
"Inverted lungs"? It isn't even just the lack of proper science. Sometimes we hear inverted speech, sometimes not. Really? Come on, Christopher Nolan - you can do better than this. Time does not affect biology. Even a 10 year old would know this.
Can effect actually precede cause without affecting reality as we know it? It is highly unlikely. Then again, our current knowledge about reversed time and entropy is mostly speculation, and the makers of Tenet are apparently generations ahead of us.
If you were caught in between the timing of discovery of time travel, and timing at which the people travelled to, then you would never exist. This is one of the fundamental paradoxes of time travel. To understand, look at this diagram:
Firstly, we see the normal flow of time in Timeline X, from Point A, then to Point B and finally to point C1. Lets say that at Point C1, all the fuss about time travel is discovered, and the people decide to go back to Point B so as to inform Sator to search for the parts of the algorithm.
Here comes the interesting part. After they tell Sator all of this, the original timeline ceases to exist. Sator knows that in the future, time travel is possible. So, another Timeline Y is created where Sator knows about the algorithm and tries to search for it.
In Timeline X, Sator is just searching for Plutonium and nothing special. But, after being informed that Sator could destroy the world, the Timeline changes from X to Y, and so it's as if the events of Timeline X (from Point B to Point C1) never happened.
Since Timeline X never happened, the people from the future never sent anything into the past, and so Timeline X should exist, but does not exist, forming a paradox. This exact explanation is the basis of the Grandfather Paradox, an idea mentioned multiple times in the movie itself.
Some of the easter eggs in the movie, though, are appreciable. For instance, this man? The one in the car, hidden behind the bars:
He was in the car when Kat was walking towards the warehouse. He is wearing a mask, which likely means that he is from the future, observing them so that he can relay information to his allies in the present. A "Temporal Pincer Movement", if you will.
It is unlikely that he is one of Sator's men, why would he observe himself? No, he is probably one of the future Protagonist's men, sent to observe his decisions.
Do you have any clue as to why sometimes we hear inverted speech, and sometimes not? Let us know in the comments below.