Although I don't see other of its components, this could be the cause:

3rd Party Preference Panes: ⓘ

    Flash Player (installed 2017-12-14) [Lookup]

    Rapport (installed 2012-04-25) [Lookup] <<<<<

Rapport has never worked properly on Macs in spite of what appear to be honest efforts on the part of the developer to cure its ills. Plus your version will be six years old soon. Completely remove it using the developer's instructions here:

http://www.trusteer.com/en/support/uninstalling-rapport

Adding more RAM might be appropriate but, at the time of the test, your iMac was not starved for RAM. Therefore adding RAM will not miraculously make your computer twice as fast. Yours can hold up to 32GB RAM but I suspect adding two 2GB modules (to make a total installed of 8GB) might help a little. We have an iMac and a MacBook Pro of the same time period as yours but with slightly slower 2.5ghz i5 processors. They have no issues running well with High Sierra and 8GB RAM.