Very Slow iMac

Hi there!

Your USB 3-equipped Mac with a hard disk can have improved system performance by connecting to an external solid-state drive (also known as an SSD).

An external SSD as your startup disk can give your Mac additional performance for system responsiveness, apps, startup, and more. An SSD will make your Mac 10 to 40 times faster for storage, and in turn, make your entire Mac faster and more capable.

To learn more, see Use an external SSD as your startup disk … - Apple Community.

Jack

the bottleneck is likely

1.00 TB (Mechanical - 5400 RPM)

mechanical harddisks are snails compared to the SSD disks new computers use and the 5400RPM is the slowest mechanical hd one can get