Unless you are doing string interpolation to generate JSON, it is absolutely more resources... and how much will depend on the framework(s) in use. To/From JSO

Unless you are doing string interpolation to generate JSON, it is absolutely more resources... and how much will depend on the framework(s) in use. To/From JSON is a straight serialization/deserialization, where templates are a bit more involved. It's not a crazy amount of overhead... and even then, it's a matter of how interactive the site/application is or needs to be. You can have a horrible or great UX in either direction in different use cases.