The likelihood that you have any viruses on your Mac is pretty close to 0% as there are currently no know Mac viruses in the wild (last I checked). What is more likely is that the AV software you downloaded mistook normal, benign files as viruses. There are other types of malware that can cause problems.
Start by downloading an antivirus program like McAfee Total Protection that has Mac-specific antivirus capabilities. Once installed, you will be able to scan your Mac for threats in a variety of ways, including on-demand, in real-time and via a schedule you set.
Download TotalAV free Mac antivirus software 2020. Stay 100% safe from malware and viruses with TotalAV free antivirus for mac. Please download the TotalAV app by clicking on the Mac icon below, and then locate the downloaded TotalAV.dmg file from your ‘downloads’ folder (most likely in the downloads section of your browser).
You don't need AV software on a Mac. While you have have some malware on your computer, it's not the first thing I'd consider if the computer was running slowly. To start, I'd look at Activity Monitor and see what's going on. AV software itself is notorious for slowing down Macs.
For more in depth diagnoses, use Etrecheck.
Feel free to post the results here so people can help you figure them out.