Thanks for the report.
Failing hard drive - This computer has a hard drive that appears to be failing.
The reason your iMac is slow is due to the 5400-rpm hard drive, and it also appears to be failing! Luckily, you can use an external SSD as your startup disk to run macOS and all your data from that. It will make your Mac 10 to 30 times faster for storage, and in turn, make your entire Mac system much faster. For more info, instructions, and what external SSD to buy, please see: How to Setup and Use an External SSD as your Startup Disk on a Desktop Mac - Apple Community.
Another option is to replace this Mac with a new Apple Silicon Mac: Mac - Apple. If you go this route, it is generally recommended that you purchase 16GB of Memory with the largest SSD that you can afford. You can also use Apple Trade-In to get credit towards a new Mac: Apple Trade In - Apple.
No Time Machine backup - Time Machine backup not found.
However, since your drive is failing, you need a backup! To start, you need an external hard drive. Here’s a few excellent ones sold by Apple:
You can then follow the steps in this Apple Support article to setup your Time Machine backup: Back up your Mac with Time Machine - Apple Support.
Also, moving forward, I recommend that you use iCloud in addition to Time Machine to sync all your files and photos to all your Apple devices. Although iCloud is not a "true" backup service, iCloud can also act as an emergency off-site backup, but this is why iCloud should be used in addition to Time Machine. Here are some resources on setting up iCloud for Mac:
After you setup iCloud, turn off "Optimize Mac Storage" in System Preferences > Apple ID/iCloud. That way, all changes made to files in iCloud are also backed up to Time Machine. If you have this setting on due to a limited amount of storage on your Mac, that is fine. Just know changes won't be backed up to Time Machine, only iCloud.
Cheers,
Jack