Question about using 27" iMac as External Monitor for new Mac Mini with a Luna Display

My 2019 27" iMac with I9 and SSD is very fast. No issues with Monterey or Sequoia speed. I run Monterey on my internal drive and Sequoia on an external TB drive. If your iMac is slow perhaps it is from some app or other (antivirus, etc.) that is the cause. You can run the free app Etrecheck and see if any issues.

See my reply in your duplicate thread:

27" iMac as External Monitor for new Mac … - Apple Community

Regards.

Re varjak paw's answer in your other thread: this screenshot is from Luna's own support page:

The red emphasis is theirs, not mine. Lag will also affect keyboard response. If you are accustomed to instant on-screen response to keyboard input, you may be very disappointed. I see no way that this and similar "solutions" can give a proper user experience when driving the primary display.

I strongly agree with your posting an EtreCheck report for you current iMac. Those of us who have reviewed thousands of them here may spot something slowing your iMac that is fixable at home. At least you would have a data-based evaluation of your current situation to help in decision making. You may be able to get extended service from it. My 2017 iMac 5K with factory SSD is still wicked-fast.

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

https://discussions.apple.com/docs/DOC-25000021

If you have any external HD's or other peripherals, they can have issues too and cause performance problems with the Mac. The most common thing we see on these forums is external HD's causing problems. An easy check is to unmount and physically disconnect any external HD's temporarily and then restart the computer in Safe Mode and then restart normally and test the computer. If the performance returns you likely have found the problem, the external HD. To confirm it's a failing External Hard Disk download DriveDX and test the suspect drive. If ANY errors appear, then you have confirmed it is failing and must be replaced.

Posting an EtreCheck report is also a smart thing to do too.

I would just buy a nice 4K external display (about $600) and add that to the MM. Why, simple:

• No expensive software to install, maintain, pay licensing to.
• No expensive cabling necessary
• Simple setup
• Dead Reliable

You can try Luna display if you would like however I think you will be more satisfied with a real external display.

Fair enough.

FYI, the prime causes of slowness in recent iMac are:

• Mechanical hard drives. Newer macOS versions seem optimized for faster storage options. The advantage to EtreCheck in a case like yours is that it tests the hard drive speeds, often pointing to serious, even data-eating issues.
• Useless utilities. Anti-virus, so-called "cleaning" apps, VPNs, and other fearware foist upon Mac users can be performance killers. Apple builds elegent self-maintenance and protection routines into every copy of the Mac OS so we don't have to give money to scoundrels.

If you storage tests slow, there is an inexpensive workaround that can be done at home, often for under US$100.

A non-software test for drive issues:

• Is the computer slow to boot up from cold?
• Are big apps slow to launch?
• Once launched, do those same apps seem to function at acceptable speed?

Three "yes" answers should bring storage into your troubleshooting sights.

If your i9 iMac is too slow using Luna will not be a good solution. It's been reported as jerky and slow (this is 3rd hand so take it for what it's worth). My choice would be (and is when I upgrade) would be a M4 Mini configured as you need with a 32" LG 4K monitor ($350 @ Amazon).

Just some food for thought.