You don't need to read the manual, or even any tutorials, IMO, as long as you know that function application in Lisp looks like (f arg1 ...), know how to program, know that Lisp has macros, almost everything else follows.
It took me a few hours to learn enough Elisp to customize .emacs, add custom macros, and start using defadvice to wrap existing commands with extra behaviour, like preserving the selection for text replacement in a selected block. I even went as far as writing a macro that could use defadvice to generically preserve the selection for any other command on the second day of using emacs.
A bigger hurdle was learning the Lisp jargon, so that I could find the correct function to call when searching for it, whether in emacs itself, or online. But once I knew about the point, that the selection is called the region and is the text between mark and point, that files are called buffers, that open is called find, etc., it was pretty smooth sailing.
It's fairly trivial to find the source for an arbitrary elisp function - just C-h f , and follow the source link. Plugins like helm (dynamically filtered list searches) make it very easy to find relevant functions too.
Elisp has a couple of features that make it work well as an application extension language: dynamic variable scope and function advice are two that are fairly unique to Lisp (technically, Javascript has dynamic scope via the eval function, but it's not usually a good idea to use it). These things make it easy to tweak behaviour from the outside.
However, contra other people's opinions, Elisp is not fast. It is slow, very very slow, compared to efficient languages. My startup time is just about tolerable on a modern machine, but not tolerable enough to stop me starting emacs --daemon every time I reboot. Scrolling through large files with line numbers enabled is slow, and the UI has a few milliseconds of lag for almost every non-trivial action. Dynamic searches need very noticeable debounce delays for list refreshes or otherwise things get unresponsive.