bug#30854: 27.0.50; Speeding up package.el startup

[arthur . miller]

> There are many ways to make Emacs's startup slow.
> One of them is to have many packages installed.  The growing popularity
> of ELPA means that it's now common for users to have hundreds of ELPA
> packages installed, which will easily cause Emacs's startup to take at
> least 1s even with an empty ~/.emacs.  For users like me who (re)start
> their Emacs session only rarely, this is not an issue, but for others it
> can be an annoyance that's significant enough to try and circumvent it
> by not using package.el (e.g. installing all their packages by hand or
> using other packaging like DOOM).
> 
> If there's no objection, I plan on installing the patch below which
> below lets users cut down package.el startup time by skipping package's
> initialization and instead loading a single precomputed file which is
> the concatenation of all the installed <pkg>-autoloads.el.  In my
> experience this speeds up activation of package.el by a factor 5.
> We could speed it up even further by byte-compiling this file, but this
> has bumped into some corner cases problems so I'm sticking to
> a non-compiled file for now.
> 
> 
> 
>         Stefan

Which corner cases apply to this? If it is OK to ask you?

I have been playing with my own init, and I see quite a speed-up by
compiling and baking package-quickstart.el into my init file (with
disabled package-quickstart).

I have also optimized it slightly: every autoload file adds it's own
path to load-path via add-to-list at startup time. But all those paths
are known when package-quickstart-refresh runs, so we can simply
concatenate them to one list and then emit code to just nconc emitted
that list to load-path at startup (this is what I do in my init file).

Together with that optimization and byte-compiling init file with baked
package-quickstart.el I see quite a speed-up, compared to non
byte-compiled package-quickstart.el as it is in Emacs.

Init time for daemon process decreased from ~0.06 to ~0.02 by just doing
this. My Gui emacs starts at almost same speed as Emacs with -Q flag
with this. My init file is not the worlds largest, currently I have only
~60 packages but it might be of interest to other Emacs users if those
corner cases were maybe worked out so that package-quickstart.el can be
byte compiled by default?