Been referring to this specific forum topic "iTunes 12.3 with el Capitan not responding" of 18,000 views, 162 replies (and counting) since it was started October 2015. This forum topic seemed to be somewhat related to my own problem with iTunes.
Not counting all the many, many other different problems mentioned in the hundreds of other topics flying around this user-based support forum.
The main issue – iTunes being unacceptably slow to respond – is an issue that has been around before El Capitan.
Almost everything you do in iTunes – create a new playlist, move a playlist into a playlist folder, rename a playlist, you name it – takes 15 to well over 20 seconds to process from beginning to end.
As of today December 30, I upgraded to the latest version of El Capitan and the latest version of iTunes. Neither of those actions have fixed the exceedingly unworkable slow behavior of iTunes on my system.
My computer setup, both hardware and software, are in very good order. This is why I have to conclude that the problems I've noticed with iTunes must be a result of changes in the iTunes application (and its interaction with the Mac OS).
When I first started using iTunes around 15 years ago, it was a good media manager (and pretty much the only one). Fairly reliable. Normal response times.
As a loyal Apple customer since 1985, I decided to follow my trust and use iTunes as my main resource for storing and organizing a massive CD and vinyl music library.
Now, after countless hours of work creating a very large library of detailed playlists, I regret that decision. For others who have created large iTunes libraries (and there are many of you out there), I am fairly certain you are having the same bad experience.
From reports on this forum topic, even people with modest iTunes libraries were having problems. (Excluding, of course, those folks who may have inadvertently goosed their systems in some other way.)
Over the past couple of years or so the performance level of iTunes has suffered a sharp decline, in proportion to the exuberant transition of that application from a pretty good media manager into a bloated software online media-selling vehicle. Shame on Apple.
Suggestion to Apple:
Split iTunes into two separate applications.
One as a robust, performance-driven, standalone media manager database sans anything even slightly related to the marketing enterprise.
The other as a dedicated Apple Media Store program.
iTunes, in its present iteration, doesn't work properly and has more problems than the Apple Support Community forum can handle.