According to your description and screenshots, part of the reasons for the iMac running extremely sluggish are obvious, including the old iMac being worn out, you haven't made any upgrades on the Mac, running too many memory-intensive apps, too many tabs in Chrome, etc. Some easy solutions may help fix the issue.
When you find the Mac running slow, you can apply these proven methods:
Quit apps and browser tabs you're not using. If there are too many memory-intensive apps running in the background and too many browser tabs are opening, they will eat up high CPU and slow down your Mac.
Free up more disk space. If there is no sufficient storage space to run programs and processes on your Mac, it also causes the Mac to run sluggish. To clean junk files on Mac, you can use iBoysoft DiskGeeker to quickly scan and remove unwanted files on Mac.
Manage your login items. When the Mac boots up, it loads the system and login items. If you have added too many login items, then it takes a long time to load up the device.
Update your Mac and apps. Outdated Mac systems and applications could also be the culprit when the Mac runs slow, they drag the Mac's performance down. Therefore, it is always recommended to update Mac and apps to the latest.
Uninstall unnecessary apps. If any unused big apps are on your Mac, they occupy Mac memory and slow down the performance. Besides, some adware apps could also be the reason. You can uninstall these apps with ease by right-clicking using iBoysoft MagicMenu.
Boot your Mac from an external drive. You can also make a bootable installer for macOS and boot your Mac from it if the old Mac hard drive is worn out.