hard drive too slow

You're welcome.

While it IS possible to replace the internal drive, it requires that the iMac be torn down. That requires special tools and a certain skill level and has a high potential to otherwise damage the computer. Apple will not upgrade the drive. If the drive were to fail, they would only replace it with like parts. An Apple Authorized Service Provider might do so, but could refuse due to the additional risk. It's easier and cheaper to use an external startup drive.

The requirement for using an external drive is simply to buy a compatible drive and cable, install macOS on the drive and away you go. Quickly! 🙂

Good luck, my friend. Post again if you need additional guidance.

This...

Drives:

disk0 - APPLE HDD HTS541010A9E632 1.00 TB (Mechanical - 5400 RPM)

And this...

Performance:

System Load: 1.95 (1 min ago) 1.67 (5 min ago) 1.59 (15 min ago)

Nominal I/O speed: 1.44 MB/s

File system: 106.20 seconds

Write speed: 51 MB/s

Read speed: 57 MB/s

The slow rotational HDD is the bottleneck in your Mac. The fix is to use an external SSD boot drive as I described in answer to your other post. Read and write times with an external SSD will be many times faster than what your internal drive is achieving.

Mac External and Internal Drives and Enclosures - eshop.macsales.com

I see no other major causes for concern in your report, my friend.

+1

The reason your iMac is very slow is due to the 5400 rpm hard drive. If you use an external SSD and run macOS from that, it will make your Mac 10 to 30 times faster for storage. For more info, instructions, and what external SSD to buy, please read: How to Setup and Use an External SSD as y… - Apple Community.

Do not replace the internal drive.

Cheers,

Jack