late 2015 iMAC running very slow.

We have nothing really to go on so please do the following:

It will help us get a good idea of what is causing your problem if you provide a report of your system, that way we do not have to play 20 questions with you. The report we are requesting does NOT provide any personal information and is extremely safe to use. Please navigate to www.Etrecheck.com and download the free version of EtreCheck. Once you have you downloaded the app and installed it, please run the report and save it. This report will help us get a good idea what has been installed on your system and help us be able to diagnose what may be wrong.

When you have your report, you can attach it when you reply to this message and we can then review it and help you determine what is needed to get your system running well again. 

For instructions on how to download your EtreCheck report and attach it to your reply to this message please click https://discussions.apple.com/docs/DOC-250000211

I'm using my 2015 iMac to write this, which is also connected to dual Thunderbolt displays for a total of three displays. Performance is fine so long as it has plenty of RAM and an SSD storage.

When I see these kind of posts, it almost always is because the machine is lacking sufficient RAM and almost always has a mechanical hard drive instead of an SSD.

As others have recommended, post an EtreCheck report so we can see exactly how your system is configured. Your OP is lacking details.

+1 - Please post an EtreCheck report.

Jack

I have that exact model. First you need to test, but ultimately I found a dismally slow HDD to be the bottleneck. An external boot SSD Brough my Late 2015 21.5" Retina up to being a joy to use.