MBP freezes for 30 seconds all the time since this afternoon I'm looking to find the cause why I'm getting the spinning beachball on my MBPro all the time since this afternoon.
I can work for like 30 seconds and get a spinning beachball for like 30 seconds. During that time OSX freezes. I can move the mouse cursor but click nothing. When I write something on the keyboard it is processed after the freeze.

Here's what else changed:


booting takes like 10 minutes (compared to 20 seconds normally)
Stop sign when power up the Mac
can't launch in safe mode
no start sound


How can I investigate the issue? Everything seems to work but it is awfully slow...
I've got a 480GB SSD and a 750GB HDD (in the CD Mount). The MacBook has 16GB of RAM and is running the Gold Master of El Captain.
@Tetsujin is completely correct about the disk. My first hope is that you have a recent backup. If you don't, try to get one, but if the disk is actually the source of the problem, getting a new backup may be difficult or lengthy.

If you don't have a recent backup, you Time Machine with an external hard drive will produce one for you. My preferred first-line-of-defense backup, however, is a clone, which I create with Carbon Copy Cloner. Once this software has successfully created a clone, you should be able to boot from it with little noticeable difference from booting from the internal drive (other than, hopefully, a lack of trouble).

Once you have a clone and have booted from it, if that confirms that the problem has been resolved you can be pretty sure your hard drive is the problem and take steps to replace it.