iMac takes too long to open apps

My father has a 2019 iMac (with latest version of macOS installed) and about 800GB of free space. The Mac is taking too long to open apps. First, when you click on an app nothing initially happens. After a few seconds, the icon starts bouncing and after minutes, the app launches. After opening and closing the app, it launches quickly. What is the problem? This is the Activity Monitor while opening Word. Also I just found out that when the wifi is off every thing is ok and apps run fast.