My mac got too slow within certain minutes

Question

Hello,
I've got a huge problem with my iMac: -
iMac from 2012
8GB Ram
3,4 GHz i7 Quadcore
1 TB hard disk
AMD HD 6970M 2GB
- within only minutes, my mac got unbelievable slow. Way too slow.
Certain examples:
My mac isn't able to show the clock on the dashboard fluently, that means, the second hand does two steps, and then the mac stops for 10-15 seconds. The again two steps, then it stops again..
My mac is useless. After every click with the mouse, the colored circle comes up, and I have to wait for 10-15 seconds.
I didn't change anything when this problem comes up. On the harddisk is more than 25% free space.
I am absolutely confused and frustrated.
I should tell you, that my windows partition on the Mac hard disc works without any problems, so actually the problem cannot be any hardware issues... I think..
Please, Please help me, I need my mac.
Greetings
Tim

Answer 1

Launch the Console application in any of the following ways:
☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)
☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.
☞ Open LaunchPad. Click Utilities, then Console in the icon grid.
The title of the Console window should be All Messages. If it isn't, select
          SYSTEM LOG QUERIES ▹ All Messages
from the log list on the left. If you don't see that list, select
          View ▹ Show Log List
from the menu bar at the top of the screen. Click the Clear Display icon in the toolbar. Then take one of the actions that you're having trouble with. Select any messages that appear in the Console window. Copy them to the Clipboard by pressing the key combination command-C. Paste into a reply to this message by pressing command-V.
The log contains a vast amount of information, almost all of which is irrelevant to solving any particular problem. When posting a log extract, be selective. A few dozen lines are almost always more than enough.
Please don't indiscriminately dump thousands of lines from the log into this discussion.
Please don't post screenshots of log messages—post the text.
Some private information, such as your name, may appear in the log. Anonymize before posting.