The 'startle effect' is the industry term for amygdala or limbic hijack. In very simple terms we react to things in two broad ways:
1) Stimuli - senses - sensory cortex - amygdala - response. This is the cognitively engaged way; we are thinking about our reaction and it can be as low as 250 milliseconds for racing car drivers or professional athletes etc. But normally it's 500+ ms.
2) Stimuli - senses - - - - - - amygdala - response. This is amygdala hijack; it's reflexive and happens as quickly as 14 milliseconds. 14 ms!!! Too fast to stop or comprehend.
All mammals have this instinct and pilots are mammals. Some folk may be more resilient to amygdala hijack than others but this it not assessed directly at any point in pilot training/recruitment.
The problem is that this all occurs basically instantly HOWEVER the effect on cognitive performance can last for up to 60 seconds. Especially at a time when pilots are required to perform complex tasks with multiple stimuli - go-around with ATC instructions and conflicting traffic.
Also the research indicates that those who are highly proficient at a skill actually react better during amygdala hijack and those who are low in proficiency react even worse. So all the below average pilots who are probably less resilient to startle and don't mentally prime themselves well are affected even more than the average by 'startle'.
Also also; current training regimes can do almost nothing to really replicate real world startle effects. So in the sim you might see a pilot react poorly for a few seconds but fix it. And in real life you see what happens in this case....