Hi. I was having this same problem. Lots of spinning beach balls all the time. I have 4 different Macs and upgraded all of them to El Capitan. They all have the same basic setup. Same applications etc. But my early 2011 MacBook Pro go the beachball problem and ran super slow. My other 3 Macs were fine.
My 2011 Mac would slow down on everything. For instance, at one point I tried Force Quit a non-responding program. I clicked on the Apple menu, but the computer froze on the Recent Items menu. It was trying to load that and couldn't. I could not even move past that menu to get to Force Quit. I had to hold down the power to restart.
I tried the various solutions posted here. Reset PRAM. That didn't seem to do anything; my computer stayed slow. My mail logs were not large and didn't seem to be the problem.
Using Activity Monitor I found that Dropbox was taking a lot of CPU power. Initially it was the program using the most memory, but only 15-30%, so I didn't think it was a problem. But later I saw it jump to 80% and then 116%. I had recently copied some large Photos libraries into Dropbox. They were older photo libraries I wanted to back up. I wasn't even using them. But they were big 40 GB and 75 GB. I believe Dropbox was taking tons of memory to sync those photo library files. Exiting Dropbox immediately brought back my computer's speed. And once I moved those photo library files back off of Dropbox, I could keep Dropbox running, but it wasn't slowing everything else down. The syncing by Dropbox of so many files seemed to be the problem.
Now my computer is running fast again. And it is basically beach ball free!
My advice is to run Activity Monitor (in your Utilities Folder). Sort by % CPU and see what is using the most memory. That is probably what is causing your computer to slowdown. It might be Dropbox or something else, but that will help you identify the problem program. Hopefully then you can update the program or do something else to solve the problem. I believe the speed problems many people have had are linked to a few programs acting as memory hogs. That was definitely the case for me.
Good luck everyone. And thanks to all of you who post in the Apple Support Communities. I have found many answers to troubling problems here over the years!