My Mac Mini is really slow and sometimes it Safari cannot access web pages

Run that post the results here

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Also describe better what you mean by slow

If you use iPhoto or Aperture, your photo library will automatically be converted to Photos format, and if you use iCloud Photos, a large amount of data may be uploaded to Apple. That will slow down Internet access for some time.

Launch the Activity Monitor 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 and start typing the name.

Select All Processes from the View menu, if not already selected, and also select the Network tab.

Click the heading of the Sent Bytes column in the process table to sort the entries by bandwidth usage. You may have to click it twice to get the highest value at the top. What is it, and what is the process?