Sequoia download - or any other way to speed up imac

You do not mention what type of storage you have, nor how full it is.

These models came with either a "Fusion Drive", or an SSD.

The former is much slower, and it is quite possibly rather full, which can make it slow as molasses in January.

Also, what software you have installed can play a part.

If the problem is indeed a slow and full-ish drive, you might get some relief by installing an external SSD, installing the OS and booting from it.

But before spending any money on this now old mac: Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

By looking at the report we will know, including possibly some third party stuff that may be running and causing some issues.

To address the current slowness while using macOS 13 Ventura with this machine

iMac (Retina 5K, 27-inch, 2017) - Technical Specifications

Part 1 of 2

Restart the computer in Safe Mode 

Do the issues persist ?

Sometimes a Safe Boot followed by a Normal Boot will just put things right.

If not - there could be something in the main User Account playing up. To further isolate this - Set up users, guests, and groups on Mac. Then log out of the Main User account and log into the dummy account and test again if the issue persists.

If the issue is present in the dummy account - then, this appears to be a System Wide issue on the computer.

Part 2 of 2

To Drill Down further and to avoid a session of Q&A, Q&A  and Q&A  

Download the Application Etrecheck  ( External Link ) directly from the Developer.

The Application is Not a " Silver Bullet "  and is  only a tool to examine the Hardware / Software used on this computer 

This is a Diagnostic Tool that makes no changes to the computer Hardware / Software used on this computer 

The application is free or paid from added features. 

The Report will Not Reveal Any Personal Information. 

Post back the Full Report - copy and paste - >>>> using the Additional Text Icon ( 3rd Icon to last ) <<<<

FWIW, Ventura is the latest version of macOS a 2017 iMac supports.