malware

Welcome to Apple Support Community.

do I need to download malware? which is the best one for iMac?

I'll assume you mean anti-malware, not malware. But, no. You should not install any anti-malware software on your Mac, except for Malwarebytes for Mac if you already have malware installed.

To see why your Mac is running slowly, I believe we need to proceed with an EtreCheck report. EtreCheck is a useful App to diagnose and identify issues, both hardware and software. It is trusted by many users. Please follow the steps outlined below:

1. Navigate to: https://etrecheck.com and get the free version of EtreCheckPro.
2. Download it. It is a free and trusted app. It is often recommended here on Apple Support Communities. 
3. Open EtreCheck, and before running the report, be sure to check “Allow full Drive Access” at the bottom of the EtreCheckPro window.
4. Run the report, and put it in a post, as shown here: How to use the Add Text Feature When Post… - Apple Community. We will then examine the report for issues in your Mac.

Jack

You don’t need any such anti-malware software messing with your Mac as @Jack-19 has mentioned. 😉

Try the following:

▷ Update macOS.

▷ Try Safe Mode.

▷ Use Apple Diagnostics to test your Mac.

If that does not help, then get expert service and support at the Genius Bar.

Click ▶︎ Start a Repair Request.

Need more help?

• Call Apple Support at 1-800-APLCARE.
• International Support.
• Contact Apple Support.
• Chat using Messages.

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

IMPORTANT:

Before running Etrecheck assign Full Disk Access to Etrecheck in the first window that comes up so that it can get additional information from the Console and log files for the report:

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

Copy the report

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

Then we can examine the report and see if we can determine what the cause of your problem is.