Because will_paginate does not support mongoid skip and limit methods. They just supports paginate method in Array class. So what it means? Will_Paginate, find all objects and after that cut the needed objects. It can cause performance problems when you have a lot of objects.

In your Gemfile use gem "will_paginate_mongoid" and run bundle install

or

Just run gem install will_paginate_mongoid

It just create a paginate method compatible with will_paginate interface and add two additional criterias: skip and limit

Yes. Now you can use paginate method like in will_paginate documentation