iMac is slow, memory pressure green
I remember your other questions, so will try and help you get to the bottom of this. You need to understand though, that troubleshooting is a process of elimination and, as a result, can take some time.

Before we start, I am going to ignore that you had a virus a few weeks ago, because that could be confusing things. In other words, we need to start afresh because the virus and this issue may be totally unrelated.

I can see from the various images you posted that you do have plenty of free hard drive space, so that's one possible cause we can eliminate. So, one by one, let's try and resolve this.

1. Let's start your computer in Safe Mode

Our first test will be to see what happens when you start your iMac in Safe Mode. Doing this forces your iMac to perform some tests and also prevents some software from automatically loading or opening. 

Follow these steps:


Shutdown your iMac
As soon as you hear the startup sound, press and hold the Shift key
Once you see the Apple logo, let go of the Shift key


Don't be concerned if your iMac takes longer to boot up (remember it's performing some tests). After it's booted, use it for a while. Do this for long enough for you to be satisfied that the problem still persists (or not). Once you're done, restart the computer normally to exit Safe Mode. 

Note: While in Safe mode you won't be able to do everything you normally can, and your macOS environment may appear/work differently. Do not worry about this.

When ready, report back on how you went.

2. Create and log in to a new user account

Our next test is to create a totally new user account and check whether logging in to this has any impact on performance. In case you don't know how to do this, follow these steps:


Go to Apple Menu > System Preferences
Select Users and Groups
Click on the + sign at bottom left to add a new user (you may need to unlock the padlock below that to make changes)
Set your user options and ensure that automatic login is off
Exit System Preferences


Now restart your computer and log in to the newly created user account. Once logged in, do not install any new software etc. Instead, just use the computer for a while to determine if it is still slow.

When ready, report back on how you went.

3. Install macOS Sierra on an external drive

Our next test is to take advantage of your external hard drive to try and determine whether we have an issue between macOS Sierra and your computer. If not, it may indicate either a problem with your internal hard drive or 3rd party software.

Use this process:


Do a full installation of macOS Sierra on the external hard drive but do not install any 3rd party software.
Restart your Mac.
Immediately press and hold the Option key. 
The Startup Manager will appear and shouldn't take too long to list your internal hard drive and the external drive.
Use your mouse or the left/right arrow keys to select the external hard drive you just installed macOS Sierra on
Double-click or press the Return key to start your iMac from the external hard drive


Now use your iMac from the external drive for a while doing a few things to get a sense of how well it's responding. Please only use default Apple apps (such as Safari etc) for now.

When ready, report back on how you went.