If you have IQ 50, it's not just that you do things slower than other people, you can't do many things at all.
There are two different form of understandability that comes to my mind,
1. Things that you don't understand because you don't have the necessary background knowledge, 2. And the things you'd fundamentally never understand because of the limitations of your own mind, which is, limited by the IQ.
I' assuming you're trying to mean the second version here.