eintr: Byte Compiling
1.5.1 Byte Compiling
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One other aspect of interpreting: the Lisp interpreter is able to
interpret two kinds of entity: humanly readable code, on which we will
focus exclusively, and specially processed code, called “byte compiled”
code, which is not humanly readable. Byte compiled code runs faster
than humanly readable code.
You can transform humanly readable code into byte compiled code by
running one of the compile commands such as ‘byte-compile-file’. Byte
compiled code is usually stored in a file that ends with a ‘.elc’
extension rather than a ‘.el’ extension. You will see both kinds of
file in the ‘emacs/lisp’ directory; the files to read are those with
‘.el’ extensions.
As a practical matter, for most things you might do to customize or
extend Emacs, you do not need to byte compile; and I will not discuss
the topic here. 
Byte Compilation (elisp)Byte Compilation, for a
full description of byte compilation.
Info Catalog
eintr: Complications
eintr: Lisp Interpreter