Use Notepad++ As Offline JSON Validator
If you are on Windows & already using NotePad++, you can use it as an offline JSON validator tool. Just install this plugin. Now you can validate & view JSON text in tree structure inside the text editor. It might not give you fancy UI like some of the online JSON formatters do, but it is very performant & it works pretty well with large text. NotePad++ is well known for handling large text files & this JSON viewer doesn't disappoint you in that aspect.