Upgrade for iMac Retina 4k, 21.5-inch, 2019

That iMac is not user upgradable. You may find an Apple Authorized Service Provider willing to upgrade the Hard Drive and RAM, but you'll be stuck with the slower processor.

You're best bet, is to sell, repurpose or recycle that iMac and get a new one with the upgrades that you need.

https://www.apple.com/shop/buy-mac/imac

"Slow" in 21.5-inch iMacs models from 2012 through 2019 are, 95+% of the time, due to the slow mechanical hard drives Apple opted to use in this modes, not the CPU or RAM. The mech drive in our 2011 21.5-inch iMac tests 30-40% FASTER than the base drive in newer models wit the same screen size. Go figure. The base drive in your computer can, at its best, only transfer data at about 80MB/sec

If this is your situation, adding RAM or changing the processor does zero, zip, nada. Waste of money.

Your 2019 iMac will continue to get OS upgrades as least through Sequoia to be released the fall. If you wish to keep it in inservice longer, there is a cost-effective, at-home solution that will, in its least expensive incarnation, do data transfers at the least 400MB/sec and make you think you have a new computer. It is the external solid-state drive.

See: Use an external SSD as your startup disk … - Apple Community

A more expensive version of the same solution using a Thunderbolt SSD can give transfer speeds over 2000MB/sec

Allan Jones wrote:

"Slow" in 21.5-inch iMacs models from 2012 through 2019 are, 95+% of the time, due to the slow mechanical hard drives Apple opted to use in this modes, not the CPU or RAM. The mech drive in our 2011 21.5-inch iMac tests 30-40% FASTER than the base drive in newer models wit the same screen size. Go figure.

Between 2009 and 2011, 21.5" iMacs had four user-accessible RAM slots, built-in optical drives – and, I believe, enough room inside for a 3.5" desktop hard drive.

In 2012, when iMacs became really thin, the 21.5" iMacs gained Thunderbolt 1, USB 3, and SATA 3 - but lost

• Half their RAM slots
• User-accessible RAM slots (the remaining two were sealed deep inside)
• Built-in optical drives
• FireWire 800
• The room to accept 3.5" desktop hard drives

Now they shipped either with slower 2.5" notebook hard drives or with SSDs. Often people buying 21.5" iMacs because they were the cheapest iMacs didn't spend extra money on the SSD option.

...and, I believe, enough room inside for a 3.5" desktop hard drive.

You are correct. I can confirm the HDD in our geriatric 2011 is 3.5-inch, 7200 rpm. It is an SATA 3 drive on a SATA 6 bus but still posts speeds in the 90-110MB/sec range even after all these years.

And considering my iMac 2017 5K has the factory NVME SSD option and my old Macbook Pro has an SATA 6GB SSD conversion, yes the old iMac feels slow. Better than 70-80 MB/sec from an SATA 3GB 2.5-inch 5400rpm drive though!

in 2012 the 3.5-inch drives in 21.5-inch iMacs were brutally sacrificed at the Altar of Minimalist Design.

+1

Yep, thinness added nothing but misery.