Which OS best for iMac 21.5 Late 2015 1.6 GHZ Dual-Core Intel i5 with 8 GB

Hello,

As Allan said, you should consider using an external SSD as your startup disk. While this hard disk found in your Mac is perfectly suited to light tasks, such as email and web browsing, but for more advanced tasks, and for users who want the maximum performance from their Mac, this hard drive does have its limitations. However, you can use an external SSD as your startup disk to run macOS and access your data. It will make your Mac 10 to 40 times faster for storage, and in turn, make your entire Mac much faster and even better.

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

Jack

Welcome!

You wrote:

I have the mac listed above with 1.6 GHZ Dual-Core Intel Core i5 with 8GB 1867 MHz DDR3

That is the crippled educational/institutional iMac that benchmarks at rough 1/2 the speed of the standard consumer 21.5-inch iMac, the iMac 4K. Yours suffers a a slow, laptop-class dual-core processor; the 4K uses a proper 4-core desktop processor.

In short, that computer can never be "fast."

There is one cost-effective option available: get a USB3-rated external drive than contains an SATA 6G solid-state drive. Clone—not copy—the content of your internal drive to the external and then use the Startup Disk system preference to set the external as the boot volume.

That does not help the the crippled processor but the 5-6X increase in data transfer will make the computer boot faster and apps will launch in about 1/3 of the time, making the user experience more enjoyable. No one likes watching spinning beach balls!

The advantages of the external USB boot solution are several:

• You can do it yourself at home
• You can do it for under US$100.
• The old internal drive remains for extra storage.
• You do not have to open a computer whose case Apple sealed shut
• The external SSD will still be usable fo external storage when you get a new computer.

24bit_192khz_Music wrote:

Can you not upgrade the RAM ?

RAM in 21.5-inch 2015 iMacs is soldered to the logic board; there are no "slots." Even the minor deities on Mt Cupertino cannot increase the RAM once that model left the factory.

and why not change the hard drive to a new fusion drive?

The required connector for the SSD part of a Fusion drive is not present unless the computer has a Fusion drive of SSD when it left the factory. Embrace the fact that Apple's incarnation of a hybrid drive consisted of TWO separate devices—a small SSD and a larger mech had drive—linked by software to appear as one drive to the user

Apple did not want the case opened, and doing so requires special tools and a reseal kit and,even then, carries the risk of destoying the computer as one cuts the factory seal.

Can you not upgrade the RAM ? and why not change the hard drive to a new fusion drive?

Think things will improve

The best os is always the latest os.