Worries because I'm slow in understanding course materials and my mind goes blank in exams I am undergraduate in Computer Science and my problem is that i don't quite understand what's wrong with me 


When I am taking a course I am able to understand it but at much slower rate then other students which results in me taking 2 or 3 courses instead of 5 or 6 that i should be taking, also I have to go in deep details for every subject to be able to understand it and this is very time consuming because it requires alot of research.
I have also noticed that my fellow students are able to read much quicker and to understand what the Author is telling them, while I often need much longer just to read the page plus I have to read it couple of times to actually understand the Author and i often get it wrong (I often get wrong what the Author actually means i.e I often interpret it the wrong way) and I am like slower at everything writing, walking, thinking, solving 
And the most devastating part is that after the exams I forget everything literally everything as if I never learned it, this is very demotivating especially knowing that in order to get my degree i have to demonstrate my knowledge from the past too.


Am I mentally retarded or just stupid (like IQ bellow 80 or something) has anyone had similar experience ?

Even if I make it will I have any chance in the workforce?
No. You are definitely not retarded. 

You mention that you spend a considerable amount of time going into deep research on every subject. Indeed one of the skills you develop (or should strive to develop) during the course of your studies is to adopt a more pragmatic approach and understand when do you need to understand every detail and when do you need to only understand things at a high level. I had a similar experience to yours so I know that this might sound to you like cheating/impostering, but the reality is that with the sometimes overwhelming load that you are faced with during your studies (and will keep facing in your future academic or professional life) you routinely need to evaluate if the gain in your overall performance from this additional rigorousness will indeed justify the amount of time you will invest in it, often at the expanse of other tasks. 

Two things that helped me personally to control this tendency are:


Studying with other students. This can help give you some feedback on whether it is indeed vital to go into that level of detail on that particular task.
Deciding in advance how much time I'm going to dedicate to a specific task. This is indeed challenging and might also be stressful at times, but knowing you have this time constraint can help you to prioritize what are the most vital elements of each task.


Another equally important point: from your description it sounds possible that you might have ADHD or some other learning disability. I recommend that you will approach the unit in your university in charge of supporting students with learning disabilities. They will be able to advise you how to optimize your learning process and help you get additional assistance that you may be entitled for.

Last but not least, have faith in yourself! the fact that you are taking such a challenging program and are studying diligently and the very fact that you asked this question shows that you are capable!