iMac 27 2019 so slow after installing Ventura it is unusable

If you can't get the download and Etrecheck running try this: boot into Safe Mode according to How to use safe mode on your Mac and download and run Etrecheck from there. Reboot normally when finished and the report posted.

NOTE 1: Safe Mode boot can take up to 2 - 3 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

The report is right. It appears you have a failing hard drive:

Write speed: 8 MB/s

Read speed: 50 MB/s

Get an appointment at the nearest   Apple Genius Bar or Apple Authorized Service Provider. The Apple store will provide a free diagnostic check. Not sure bout the AASP.  If you have AppleCare you should be covered.

i can see your mac computer's specifications page here :

iMac (Retina 5K, 27-inch, 2019) - Technical Specifications

( i think, you customized during initial purchase )

this mac computer can use max total 64 GB RAM , so upgrade RAM into that.

you will need 4 pcs of 16GB DDR4 2666MHz SODIMM RAM Modules.

16GB modules are not over-costly now.

if you replace HD-Fusion drive, with same another new one,

main problem still exists & will exist : HD-Fusion drive is a HYBRID category drive, with small amount of faster SSD, & majority/larger size of drive is slow HDD based.

HDD (mechanical & magnetic) are slower than SSD (electronic semiconductor circuitry based).

SSD can be 3 to 10 times more faster than HDD.

HDD type of drive's memory bit can crash more often under environmental stress conditions, such as over-heat, sub-zero temp, high-altitude, etc etc etc, and very less tolerant to SHOCK or DROP from higher elevations.

HD-Fusion or HYBRID category/type of drive, will again soon FAIL.

it happens more often when you have HDD.

SSD has extra storage pool memory, when a used storage memory bit becomes faulty, then SSD logic circuit allots another fresh bit from that pool.

please get a full SSD drive, 2TB SSD drive, SATA-III based.

that will make your computer working very fast, and run in stable manner for very long time.

if SSD drive quality is better, then it will perform better for long time.

as the computer has two Thunderbolt3 ports, if your monitor does not have a TB3 or TB4 port, then buy a converter/cable/adapter that can convert monitor's DP (or HDMI) port into a TB3 port based cable, then video will work even better.

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Download and run Etrecheck.  Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

DanLTaylor wrote:

Should I just go ahead and replace it, then don't need to worry about it happening in the future.

Yes, I would get it replaced. If it's not under AppleCare I would replace the Fusion Drive with a 1-TB SSD internal drive. You may have to go to an AASP for that instead of Apple.

Then I would order a couple of 4 GB RAM modules from either Crucial.com or OWC (MacSales.com) to raise your total to 16 GB. That's what I have and have never needed more.

If you are under AppleCare take it to Apple and get the fusion drive replaced under warranty. Then get and install the additional RAM yourself.

yes, you can ofcourse reset to factory settings, then reinstall macOS : see more info in this earlier discussion-post.

Hello, I’m having the same problem on the exact same model. Prior to updating my Mac was fully functioning and fast. Now it’s totally unusable. Did you find any fix ?