increase ram

Yes it will work. Make sure to purchase quality ram from either OWC or Crucial, don't use any of the bargain basement brands as Macs require exact spec ram.

While increasing ram will speed most things up if your iMac is experiencing a sudden slow down there may be something else wrong, it might be helpful if you'd run and post an Etrecheck report https://www.etrecheck.com/

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   

That being said you've installed what most of the experienced contributors to these forums consider akin to malware: MacKeeper. Uninstall it according to the developer's instructions.

Also AVGAntivirus is totally unnecessary and contributes to poor system performance. Uninstall it according to the developer's instructions.

You can check to see if you've removed all of the supporting files by running Find Any File to search for any files with the application's name  and the developer's name in the file name.  For the software mentioned you'd do the following search(es): 

1 - Name contains mackeeper

2 - Name contains avg

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

The best antivirus, cleaning app, and overall maintenance app for Mac OS is Mac OS itself. All you need to do with Mac OS to keep it secure and running well is to keep it up-to-date and do periodic (about 1x per week) restarts. Other than that, leave it alone. Adding third party antivirus, cleaning, security and other types of maintenance apps to Mac OS adds no additional level of security. The only thing these apps do is have the opposite affect users want. They make Mac OS slow, unstable, generate odd behavior (much like you are experiencing) and make Mac OS appear buggy. 

Please locate the developers uninstall instructions for AVG & MacKeeper and follow to the letter. Then restart in Safe Mode per the directions in Use safe mode to isolate issues with your Mac and then restart normally. 

It won't increase the boot speed or the speed of opening and saving apps or files. However, it will help in processing files once they are opened.

As Mike mentioned there could be other reasons for slow system boot and perfoermance. Follow his recommendation of running Etrecheck and post the results as follows:

Copy the report

and use the Additional Text button to include the report in your reply.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the Etrecheck's Privacy preference pane so that it can get additional information from the Console and log files for the report:

Also click and read the About info to further permit full disk access.

Then we can examine the report and see if we can determine what might be causing the problem.

I have to agree with rkaufmann87 about buying direct from OWC, the ram sold by Amazon is likely the correct ram but you have a guarantee that it will work and tech support second to none buying direct. If for some reason you can't get the ram from OWC don't be afraid to use Crucial ram, they also guarantee compatibility and are competitive in price.

Only buy OWC direct from macsales.com, do not buy from third parties such as Amazon!!!!

They do, but I'd buy directly from an authorized macsales.com reseller. Here is a link to learn where they are:

OWC Europe

Just click the map shown.

Install in the empty slots. DO NOT overthink this it is so simple some people over think it and make it into a THING when it's just install and run. 3 minutes and you are done.