First, there is no reason to ever install or run any 3rd party "cleaning", "optimizing", "speed-up", anti-virus, VPN or security apps on your Mac. This user tip describes what you need to know and do in order to protect your Mac: Effective defenses against malware and other threats - Apple Community.
There are no known viruses, i.e. self propagating, for Macs. There are, however, adware and malware which require the user to install although unwittingly most of the time thru sneaky links, etc.
Anti Virus developers try to group all types as viruses into their ad campaigns of fear. They do a poor job of the detecting and isolating the adware and malware. Since there are no viruses these apps use up a lot of system resources searching for what is non-existent and adversely affect system and app performance.
There is one app, Malwarebytes, which was developed by a long time contributor to these forums and a highly respected member of the computer security community, that is designed solely to seek out adware and known malware and remove it. The free version is more than adequate for most users.
That being said uninstall F-Secure according to the developer's instructions. Then rerun Etrecheck to make sure of the the F-Secure supporting files have been removed.
Another thing to remember is that you have the second slowest iMac sold by Apple at the time of purchase: 8 GB of RAM and a 5400 rpm drive in the Fusion drive.
Booting should be quick but opening and saving files on the HD will be a bottle neck.
