my imac late 2015 is very slow in using, how can i speed it up?

Hi there!

If you think your Mac might have an issue, you can use this procedure to help gather information on your Mac which will help us troubleshoot your Mac’s problem.

To start, let us know what model your Mac is and what version of macOS you’re using. The simplest way to find out is About This Mac, available by choosing About This Mac from the Apple menu  in the upper-left corner of your screen. 

Next, share a screenshot of your Mac’s storage information. In the Finder, choose Go > Utilities, then double-click the Disk Utility icon. (Disk Utility is located in the /Applications/Utilities folder.) Alternatively, click Launchpad in the Dock, type Disk Utility in the search field, then click the Disk Utility icon. Once Disk Utility is open, take a screenshot of the information it automatically opens to, which will be your startup disk. Next, select View > Show All Devices in Disk Utility and select the non-indented disk entry—it’s at the top of the hierarchy. Then, take a screenshot of the newly selected entry. Upload all screenshots here. 

Next, use Apple Diagnostics to test your Mac to help determine which hardware component might be at fault. When testing is complete, Apple Diagnostics shows the results, including one or more reference codes.

Learn about Apple Diagnostics reference codes

Let us know which reference codes you receive.

Finally, let us know what antivirus, VPN, or cleaning software you have installed. 

Jack

+1 on Jack's recommendation. The exact sub-model with processor speed is very important in diagnosing Late 2015 iMacs. There was the regular consumer version with a Retina 4k display, plus a slower, cheaper educational/institutional model that had a non-Retina display and slower processors to keep costs down for bulk buyers.

One of the slower non-Retina models was downright crippled with a pokey laptop-class 2-core 1.6ghz processor. It was awful, with benchmarks that were only 40 percent oof the next higher model with a faster 4-core processor.

If you have the 1.6ghz version, it will be hard to get that one "out of low gear."