MacMini mid 2011 very slow with latest OS X version. Which Version is best solution for me?

Give the computer some time to finish indexing your disk. It can take a couple of hours if you have a lot of files. That Mini should run both 10.10.x and 10.11.x just fine.

 

Get an SSD and say goodbye to the wheels.

Although 10%+ CPU usage at all times is not normal. But without more information we can't help you there.

 

What follows is my opinion only, and I've written about this numerous times here on macrumors.

Once you "go beyond" OS 10.8.5, and get into Mavericks, Yosemite, or El Capitan, things may slow down quite a bit on an older Mac that still has a platter-based hard drive inside.

The OS will run, but it will seem more like it's "walking", instead of "running".

Your options:
- Install an SSD inside. This -will- speed things up.
- Install an SSD OUTSIDE, via Thunderbolt. Again, this -will- speed things up.
- Revert to an older version of the OS, such as 10.8.5. You won't get the memory-related and other disk-intensive slowdowns that you would get with 10.9, 10.10, or 10.11.

Some of the other tricks I've tried:
- Disable VM disk swapping. It works IF you have enough RAM
- Disable Spotlight -- turn it completely off
- Disable compressed memory
- Disable AppNap
All of the above work fine for me.

Again, my opinion only.
Others will disagree.
Some will disagree vehemently.

 

Well, stop using Chrome then.

 

Well, then you just have to live with high CPU usage.

 

Its hard to say why your Mini is slow without more information.

Could you post screenshots of Activity Monitor tabs Processor, Memory and Disk?

Alternatively you could install EtreCheck, it's a software that collects diagnostic information about your Mac. Once report is ready post it here.

It is possible that slow down could be a result of bugs in El Capitan but without further information its impossible to say for sure.

 

SSD!

And is it 8GB or 2GB of RAM?