Upgrading to Ventura from Mojave on my 2017 iMac

Whatever you do I would not recommend upgrading your internal drive straight to Ventura from Mojave: it´s a huge leap from Mojave to Ventura (Mojave > Catalina > Big Sur > Monterey > Ventura), and doing so might/could cause all sorts of problems! Leave Mojave on your internal drive and, as has been suggested elsewhere, buy an external SSD. Install Ventura on it and use it as your startup disk. Just make sure you completely erase and re-format to APFS any external drive you might buy before installing Ventura. I also have a 2017 iMac, and Ventura is probably the last OS we can run! Hope this helps you! Cheers!

It shouldn't slow down much if any, but you will need to verify your apps. Ventura will not run 32 bit apps, if you still have some of those on your Mojave system, they will not work on Ventura.

click here ➡️ https://www.macworld.com/article/232735/how-to-check-if-mac-software-is-32-or-64-bit.html

Any apps that are 32 bit, will need to either be updated or replaced before you upgrade to Ventura.

Is the computer configured with a Traditional Rotational Drive ?

Is the computer configured with with a Fusion Drive ?

Either of the two above will, IMHO , take a dramatic negative Performance Hit

Yes it should function but it may not perform as Unser Mojave.

Reason, IMHO, is stating around Big Sur macOS 11 and above, the newer versions of macOS are more geared towards an ALL SSD Configuration

There are ways to extend the life of the machine by investing in a good Enclosure and good SSD Drive.

Install Venture to that drive and make it your startup drive.

The performance with 16 GB RAM will be markedly more to the Users' Expectations

I also have a 2017 iMac (27"). I upgraded to Ventura on Monday. I've run into a serious issue with external drives not only crashing my system (kernel panics), but with what I can only assume is something in Ventura has rendered one of my drives useless. Please do a search in the community and you will see mine is not an isolated incident. If you use external drives of any kind, I would recommend waiting until this issue can be addressed and made right.

See this link:

macOS Ventura is compatible with these computers - Apple Support

If may share.

Used to have a Late 2012 iMac with a Quad Core plus the 5400 RPM Rotational Drive.

Worked fine until macOS 10.14 Mojave but the moment macOS 10.15 Catalina arrived ( No more UpGrades for this machine ) it slowed to snails pace.

I explored the Apple Trade-In  Process where the Value of the exiting device was used against the cost of a new Apple Computer. 

Yup, only a few dollars but at least it did not go to the Land Fill Site

Now running M-Class machine

Food for thought ?

No, not up-selling just another option outside the External Drive Setup

Welcome and whatever directions are taken, the Forums are here to offer assistance