As I've said previously, I think this topic is more about what sort of Apple 'User' you are, rather than the age of your machine... The IT experts don't seem to have a problem, but those of us who bought a Mac because 'they just worked' (without us needing to have a clue how and why) are the ones who seem to be experiencing problems:
Unlike cars, EVERY Mac is different - there will be different hardware and software and different folder structures, options selected (or deselected), in millions of different configurations. I have to admit I do not have a clue about this - I don't have the time or inclination to learn how to avoid the dodgy software and what settings to use to optimise the software and hardware to use. All this used to be the beauty of Apple products - they just seemed to work, whatever you did, and usually warned you if you were about to do something stupid!
There are so many websites, forum posts and comments about El Capitan, there must be 'something' about it that is causing all these problems? I went through the pain of Mavericks wiping a WD Hard Drive and Yosemite glitches, but they were nothing compared to this: I have done ALL the things suggested and it is no better - I have over 30% of my HD free, memory usage (according to Activity Monitor) rarely exceeds 20%, but still EVERYTHING is slow and hangs - most times the cursor can't keep up with the keyboard and I see the spinning beachball of death more times than I don't.
OK, so I am whinging, but the people posting that their old Mac is just fine is infuriating and unhelpful: It suggests that it is US that is the problem, which is actually quite rude...
If you can't help those of us who ARE experiencing a problem (even though you don't believe us), please stop contributing to the thread and adding to our frustration - Thank you.
PS. I have read elsewhere that it was leaked by Apple that they are working on planned obsolescence, as people are not buying enough new Macs 😉 Sadly, I can't find the article again now - maybe it has been removed as I did the same Google search for it?