Can I use my iMac 2019 model as a monitor to Mac mini?

Slowness complaints about 21.5-inch iMacs made between 2012 and 2019 are quite common here. 95+% of these are due to the slow and under-spec hard drives Apple used as non-optioned storage. As the macOS evolved, those old-school drive could not keep up.

Adding RAM will not fix slow storage.

As for taking to an Apple store, know your sub-model—the bit with a year. Apple can refuse to look at older products they consider "vintage" or "obsolete."

Compare your sub-model to the vintage and obsolete models shown in this Apple article:

Obtaining service for your Apple product after an expired warranty - Apple Support

Before schlepping a heavy iMac to an Apple store, your can see if an at-home solution is available without leaving these forums. If you want a data-driven evaluation in this setting where we can neither see nor touch your computer, please post an EtreCheck report. We can quickly and within the limitations of these forums help you determine what issues are at play without your leaving the house, or our playing a protracted game of "20 Questions" with you that could go on for days. 

EtreCheck Pro is available here:

https://etrecheck.com/index

The free version will do nicely for this purpose, although the app is worthy of our financial support.

We can see hard data about drive performance, software issues and interferences, and RAM usage. Etrecheck is the development of a long-serving and trusted ASC contributor. It is a reporting app, not a "fix-it” app, expressly for displaying information in these forums to help us help you remotely. It will not reveal any personal or secure information.

Please see this excellent user tip on how to post long text reports like EtreCheck's into a forum response:

How to use the Add Text Feature When Post… - Apple Community

Yes, carongall, lanstrad1 is quite correct , it won’t help you, though it is possible thanks to Duet Display ( for which you pay) . I’d suggest not Ever downloading/using any “cleaning” apps, eg “”CleanMyMac”- they cause more problems then they solve… you could try starting up in Safe Mode which loads the basic MacOS and disables everything else .. here’s how to do that : Start up your Mac in safe mode - Apple Support (CA)

see if your iMac runs faster/better… which might suggest it’s a software/app problem… other than that, download and try EtreCheck, it’s a trusted app for finding conflicts and problems.. other than that, as lanstrad1 wrote/posted, contact, make an appointment and take it in to an Apple Store with Genius Bar, Apple dealer or AASP( apple authorized service provider)

john b

Your 2019 iMac does not support Target Display Mode. That feature disappeared when the first 27" 5K Retina iMac came out in Late 2014, and never returned. Apple later added restrictions such that no current Mac supports using any iMac as a Target Display.

Details here: Use your iMac as a display with target display mode - Apple Support

Your 2019 iMac is recent enough to be an AirPlay Receiver for AirPlay to Mac. AirPlay is not as good as a hardware-supported video connection to a real monitor (or to an iMac in Target Display Mode), and your Mac mini would need to have at least one real hardware monitor to even make an AirPlay connection to an iMac as a "second one."

Continuity features and requirements for Apple devices - Apple Support

Stream video and audio with AirPlay - Apple Support

But once you had your Mac mini and a real hardware monitor, it wouldn't cost you anything extra to play around with the Airplay to Mac feature.

You cannot use the iMac as the monitor for the mini. The iMac is an integrated system (just like a mscbook) and unfortunately there is no such feature.

Moreover: it would not help your situation with the iMac 🤷🏽‍♂️

Haven’t read other solutions, but perhaps a good clean install could help, assuming you have proper backups of DATA. If you aim to do so, i would avoid recovering entire backup and just bring back data (basically Documents, Pictures, Movies, back to their respective folders)

Process:

1) Take an external drive: copy Documents content to a Documents folder, Pictures to a Pictures folder, Movies to a Movie folder)

2) Proceed to clean install of macOS (if you have never done so, please search the topic (before this, please disconnect external drive)

3) Reinstall apps that you aim to use (not those you haven’t used in years)

4) reconnect external drive, and copy back content of your external drive (Documents to Documents, Pictures to Pictures, Movies to Movies and so on).

Test the iMac if possible in between any new app installation.

This is a long process (expect a few hours), but this way you will discard any software issue.

If this does not solve your issue, take the iMac to either an Apple Store or an Apple authorized dealer to conduct a full hardware diagnosis - they have proper tests and usually a fixed (low) rate for this assuming you’re no longer under warranty (likely the case fir a 2019).