Our body is designed to react to stress and those natural reactions can affect how easy it is to read.
Can do OK at times, but quickly deteriorates through the reading session
Difficulties lead to frustration and even tantrums or disengagement
When we become stressed the body releases adrenalin and cortisol. These have a number of effects, but a critical one is shutting down the higher brain function. You tend to simplify your reaction to things in a "fight or flight" way. There is a third reaction which is immobility, the "deer in the headlights" response.
So, if a child tends to get stressed or frustrated when things don't go well, reading becomes hazardous. Inevitably there will be words that cannot be read. So the stress levels go up and chemicals are released. Those chemicals begin to shut down higher brain function and so reading becomes harder. That leads to more stress!
We call it a "stress spiral".
It can often end up with the book flying across the room or the child sitting in a heap, refusing to continue.
The obvious way to avoid this is to prevent the stress building up. So it is always important to be extremely helpful when someone is trying to read. Don't leave them to "work it out". Get in and walk them through any word that is difficult.
You can enhance that with Guided Phonetic Reading, because the clues that let the child succeed are right there on the page.