Upgrading iMac 27'' A2115 (2019): Power Supply + Internal Drive Advice?

The Fusion Drive has been the clear bottleneck in performance.

First, please show us data on the Fusion drive. Replacing its mech component with an SATA 6G ssd could make things slower if the Fusion drive proved to be healthy or reparable.

If you want a data-driven evaluation in this setting where we can neither see nor touch your computer, please post an EtreCheck report. 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

Please post the entire report. What seems insignificant to a new EtreCheck user can hold answers for those of us who have reviewed thousands of those reports. 

Issues you face:

1— The blade SSD is located on the "wrong" side of a board for easy access. Watch this install video to judge your comfort with excavating it: https://eshop.macsales.com/installvideos/imac-27-inch-5k-2019-pciessd/

2— For each storage option they sell, OWC shows if a thermal sensor kit is required (I get all our drive upgrades from OWC), For your model they show a sensor is required if your replace the HDD part of Fusion, but not the blade SSD part.

3— a healthy or reparable Fusion Drive in that iMac model can do read speeds from 600-900MB/sec and write speeds of 1400MB/sec or more. If you install an SATA 6G SSD in place of the Fusion's mechanical drive and disable Fusion completely, you get about 500MB/sec reads and writes. That's why our seeing the Etrecheck report is so important to fully addressing your issue,

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Considering your iMac's age, even if you have to replace the power supply, by far the most cost-effective way to have excellent speed remains an EXTERNAL ThunderBolt 3/4 boot drive. They will do over 2000MB/sec if set as boot. The other plus is that when you replace your 2019—which you will—the external drive still works on your next computer.

But all this comes down to the current condition of your Fusion drive, which EtreCheck can help us judge.

I'm not sure what type of blade SSDs that iMac requires. It does appear that Other World Computing carries blade SSDs, with capacities of up to 4 GB, that are compatible with that machine.

Other World Computing – OWC Solid State Drives For iMac 27-Inch (Late 2013 - 2019)

They also carry SATA SSDs. If you do go to all of the trouble of disassembling that iMac (Retina 5K, 27-inch, 2019) to replace the power supply and the drives, be sure to make at least one bootable backup of the Fusion Drive FIRST. Afterwards, I would recommend keeping the new drives separate, rather than creating a new Fusion Drive.

Are you sure the power supply is bad? How do you know?

In my own personal experience supporting my organization's iMacs, I have never had to replace the power supply for any of our 2017-2020 Intel iMacs. Instead all of the 2017-2020 iMac issues were due to faulty Logic Boards instead. Fortunately I had a spare known good power supply for testing purposes to confirm an MLB failure. On older iMacs I would have to replace power supplies on occasion, but not MLBs. Troubleshooting the 2017-2020 iMacs is not easy due to how much they have changed.

FYI, the video link provided by @Allan Jones does not clearly show how annoyingly difficult it is to remove & reinstall the MLB. It is very easy to accidentally damage the board and even the Display Assembly itself. The video makes it look easier than it really is. Even replacing the Power Supply is tricky due to how difficult it is to disconnect the power supply cable from the back of the Logic Board (it is tight, with very little room to access the connection).

Note that only four Intel-based Macs will be getting macOS 26 (Tahoe) when it comes out this fall. Yours is not one of them. (All Apple-Silicon-based Macs released to date will get Tahoe.)

When macOS 27 comes out, it won't support any Intel-based Macs at all (source: Apple announcement at WWDC, as reported by 9to5Mac).

You might want to consider that when deciding whether to put new SSDs inside, or outside, that iMac. An external SSD would be easier to take to a new machine if, and when, you decided to upgrade.

A 1 TB Fusion Drive on that Mac would only have 32 GB of SSD space – instead of the 128 GB of SSD space that Apple included with the original 1 TB Fusion Drives.

I believe the OP's intent was to replace both of the internal drives, in which case, presumably the new blade SSD would be large enough to use as a startup drive all by itself, and would be much faster than the 1 TB Fusion Drive. But even an external NVMe SSD might beat the Fusion Drive, given that it would consist entirely of SSD storage.

You would be better served by getting a new Mac Mini 4 and a 3rd party 4K monitor. I got a Mac Mini M4 with 15 GB RAM and 1 TB SSD with a 32" LG 4k monitor w/speakers is $1418 w/tax which is $650 less than a similarly configured new 24" iMac.

With a SSD boot drive in my 2017 27" iMac it would boot power button to Desktop in a out 45 seconds.

My new Mac Mini 4 boots in 2-3 seconds.

Just some food for thought.

I have to agree with Old Toad, spending money on a 2019 iMac is not a wise decision at this point. You will be spending at least $300-$700 on an obsolete machine. Use that money towards a new M4 based Mac mini and you will have a machine that is future proofed for years to come and will be worlds faster than your 2019 could ever be. In addition you will have a 1 year warranty and it will be eligible for AppleCare.

Do yourself a favor and don't put any more money into that machine.

What you're talking about doing is going to cost more than that machine is worth.