One more question: If i do mask or someting else with point in lightroom ,how to delete it ? because there is no "delete key" on keyboard ,I know stupid question but anyway...
Usually, the delete function on Macs uses cmd+backspace, but the actual delete button is mapped to FN+backspace. If your app, i.e. Lightroom, is set to use an actual delete button, it's FN+backspace. But standard Mac behaviour is cmd+backspace.
On Apple Extended keyboards, there are delete keys

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One more question: If i do mask or someting else with point in lightroom ,how to delete it ? because there is no "delete key" on keyboard ,I know stupid question but anyway...
So, first off, I have some bad news. Those specs are of a late-2014 iMac
Admittedly they were sold up till late 2015, not mid-2015, but I didn't remember that at the time
Anyway, could I also get you to send screenshots from the "top" command in Terminal? Activity Monitor does the same, but instead of cycling between CPU, Memory, etc. top shows it all at once.
Oh, and do this at a point in time when it is lagging (assuming it doesn't do it all the time).
PS. You can take a screenshot of just an app or a section, using cmd+shift+4 (Spacebar thereafter if you want full app selection)