Slow iMac Retina 4K 21.5" 2019

That advice still holds.

There are other possible causes for slowness, such as:

• Running "anti-virus" and "cleaner" apps
• Insufficient RAM, if you are running applications that need a lot of it to run smoothly
• Failing internal hard drives

We should explore those possibilities before jumping to a conclusion in your case.

But slow mechanical hard drives, and slow 1 TB Fusion Drives (that combine a slow 1 TB mechanical hard drive and a miserly 24 – 32 GB of SSD space), are a very common cause of slowness in late-Intel-era 21.5" iMacs.

Since installing a new drive in one of those 21.5" iMacs is a tricky operation, and can run up $100+ in labor costs if you have a repair shop do it, a frequent recommendation is to add an external SSD and make it your startup drive. As long as the internal drive is not actively throwing errors that crash the computer, you can often avoid the need for expensive / risky surgery in this way.