is there a way how can i set permission for Fields like it's offered for item.
With SharePoint, you cannot set permissions on fields. The lowest level to set permissions on is an item.
You can hide fields in nintex forms - what is your exact requirement?
There is no builtin way to specify permissions in columns, as already said above.
However there are 3rd party tools that can achieve what you are trying. The only one I've seen in real life is this one
They are not free however.
and also come with a resource overhead
Indeed. As always in SharePoint every plus on permission definitions goes with a minus in performance.