How to handle keyword-value based fields

We have been dealing with key-value based fields on a need-by-basis. This mean that if the field is parsed often the individual fields are stored in their own columns and if it is accessed often enough it is stored as a string. Maybe it made sense to just store it as a string and have some code to write and read it easily.

Said this we should consider if the DB handle betters individual columns or blob of JSON instead.