This:
Drives:
disk0 - APPLE HDD HTS541010A9E632 1.00 TB (Mechanical - 5400 RPM)
Performance:
Write speed: 27 MB/s
Read speed: 43 MB/s
That model year iMac is notorious for poor performance with that 5400 rpm hdd. The drive speeds shown in your report are abysmal.
Yes, the internal drive can be replaced with an SSD but the task requires "advanced" technical skill.
It would be much easier and less risky to the iMac to install macOS on an external SSD and then boot and operate the Mac from that. Easy setup, vastly improved performance. And when you tire of the computer, dump it but keep the drive for use with the next one - not necessarily as a startup drive because it wouldn't be necessary.
Install macOS on an external storage device and use it as a startup disk - Apple Support