Taking time to understand or communicate may be mistaken for a lack of mental capacity, but having dementia for example, does not necessarily mean that someone is unable to make any decisions themselves.
Where someone is having difficulty communicating a decision, an attempt should always be made to overcome those difficulties and help the person decide for themselves.
You aren't able to stamp a label on someone that you assume has no mental capacity and you should always try to make sure that the person(s) speaks to a professional (such as a doctor) to confirm whether they need extra support and to see if that person can actually make their own decisions.
You also need to make sure that you are listening and being considerate of what that person wants also if they are able to communicate - this is why it is very important to highlight that you need to pick someone that you trust and someone that you believe will make the best decisions with regards to your circumstance.
There are multiple illnesses that can be considered as lack of mental capacity, below are some examples of where someone may need some extra support with their decisions.