Is having 150+ user fields too heavy? Our user entities have about 5 fields for the public profile, and 5 fields with settings that determine the content of 60 other fields we need for our implementation (I've thought of making this 60 "computed fields"). The last ones can be overridden by the user, so we need another 60 to store the user entered values (without loosing the defaults) and 30 fields to act as a flag that indicates if the default or custom version is used (30 flags are enough because the 60 fields go in pairs, they are 30 pairs).

Let's say that we attach the 150+ fields to user entities: Wouldn't that be too heavy, especially when viewing the user profile page (where all the other fields are not necessary)?

Should we define a custom field that would integrate a pair of the default values, a pair of the custom values, and the flag, so we would only need 30 of those within a user object? Apart from having to define the custom field, we lose the computed field module, I think. 

Should we offload user entities by creating a content type to attach the total 60 + 60 +30 fields, and link the user with the content type using the Relation module?
thats right, consider creating a new entity and relate it through a user reference field, for these types of references, the "references module" is better than the "relations module".