First you have to learn Java and the basic core Java API of course.
From there, I'll cover the most basic: straight up pure JEE web development. eg. Building a simple CRUD app with Servlets, JSP, and JDBC using MySQL.
Learning curve - a few weeks of spare time effort (eg. Evenings after work). Maybe as little as a few days if you're doing it fulltime and intensively. Working through this book is highly recommended - http://www.murach.com/books/jsp2/. After this, you'll know the basics of building simple CRUD apps in JEE, and will know your way around. Note that this is nowhere near being a master, just knowing your way around and building the simplest one trick pony toy shopfront apps.
Resources - The book mentioned above is good. There are also a lot of online resources, but the quality of those varies. The online tutorials from Oracle are fine too. Some people swear by them, but others find them too dry.
Performance and would I recommend it - Somewhat of a moot point these days since "pure" JEE isn't used much anymore. But it still holds its own for a lot of simple CRUD cases.
Don't know how relevant all this is to what you were asking, but this is essentially the first pass of learning you need to make when getting into Java/JEE. Diving straight into more advanced topics would skip over these building blocks too much, in my honest opinion.
Note: I'm somewhat biased as I'm a core Java guy (who did mostly Swing work in the past) who is currently trying to transition into the JEE space. :)