Short answer: The effect of stress and trauma on IQ varies significantly according to type and severity, but is overall fairly low relative to normal variation in scores.
IQ test development is heavily concerned with reliability (consistent scores) so while some variation is inevitable, IQ tests are designed to minimize the impact of stress:
... test-takers ...
Stress will certainly affect your test performance and cognition, see Lupien 2007.
It might be a bit hard to measure this in an experiment, as the same individuals have to take the test with and without stress. But then you have to control for some learned effect, second time the test might be easier.
As a person who has been journaling for more than twelve years ever since a very compassionate friend advised me to, I can attempt to answer why journaling seems to reduce stress.
AliceD mentioned that it's a way of writing down todos. That’s true. I also have Google Tasks for that.
The friend of mine deals a lot in algebra. We had spent a lot of time ...