Disclosing hidden disability to employer I'm doing a project-based postdoc in an European university. 

I have serious ongoing eye conditions that significantly affect my reading speed and as a result it took me 5 years to complete my PhD in a country where a standard Ph/D/ is only for 3 years. 

I have been selected for this postdoc position, I think, mostly because of my particular fieldwork and user engagement experience, which is essential for this post. So I still got hired despite my very weak publication record. And because the interview was via Skype, the PI did not discover my hidden disability, nor did I disclose it.

However, as I've been working in the position for several months now, it becomes apparent that I'm falling behind other colleagues of the team. Despite working extremely hard, I couldn't finish the work designated by the PI on time and I think I'll need a thorough explanation for that. 

The question is, will disclosing a hidden disability ruin my employment? This is a 3 year position and the initial contract is only for 1 year. Given this is an externally funded, project-based postdoc, I'm really worried that my contract will not get renewed at the end of the first year. 

And also how do I explain this to the PI and the university? For example, the choice of terms, hidden disability vs. chronic illness vs. long term health condition? Do different terms imply different levels of legal protection for employment?
In most European countries, it is usually against the law to fire people because of disabilities. So long as you're capable of performing your assigned work with reasonable accommodations, you should not be afraid of being terminated. However, you will probably need to have documentation of your disorder to provide as evidence to support your claim.