Is there a module for scalable input / output for unlimited value fields? In a field with unlimited values a user may easily enter 100+ items. While the db does allow unlimited entries, the client side input does not seem too scalable since it shows all values on the same page. Perhaps some kind of paging or Views would be great. Is there such a module?
You could of course write both a formatter and a widget to implement some sort of paging, but that doesn't change the fact that Drupal will always load all 100+ field items when loading the node and saving (by doing a DELETE first and then an INSERT) of all 100+ field items when saving that node.

There isn't really a nice way to do this properly using fields. In case of references (node, user, ..), a possible workaround is to reference those entities to the current one, so that each one only has one (or a few) references, instead of 100%. Then you can display them on the referenced entity using a view.

Using a non-relational field storage implementation like MongoDB can help with the storage performance problems because it's a single load/save operation, no matter how many fields and field items you have. But it's still one global operation for all field items at once.