Is there a module for scalable input / output for unlimited value fields?
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.