slow iMac

The problem is you bought a base model Mac that is designed for nothing more than doing e-mail and surfing the Internet and then installed professional grade apps on it. It seems that you bought the computer based on it's low price and not on what your needs are. When buying any product you should buy based on what your needs are and not just the price.

The bottleneck on 2012-2019 21.5" iMacs was the glacially slow 5400 RPM HD. There is nothing you can do to speed that up, however you can improve performance dramatically by adding an external SSD and then installing Mac OS on it and using that drive. Please buy a USB 3/Thunderbolt 3 external SSD and you will then have a suitable machine. One like this is perfectly fine:

https://eshop.macsales.com/shop/owc-envoy-pro-ex-thunderbolt-3

The next alternative is to replace the computer with a 21.5" with a bare minimum of 16GB of RAM and the largest capacity SSD you can afford.

Above answer from rkaufmann87 is correct. Those base-model 21” iMacs were about as slow as it gets, both due to the glacial hard disk drive mentioned, and due to the lack of main memory, and there’s little that can be done past what was previously suggested. Those Macs were for schools and school kids and kiosks and low-end web browsing. Adding external Thunderbolt 3 fast SSD storage can and will help, but the lack of memory will still slow those models.

What’s the speed difference mentioned here? A fast hard disk drive will do maybe 150 to 200 operations per second. This is a measurement sometimes known as IOPS, or I/O operations per second. Storage read and write requests from macOS and your apps for whatever you’re doing, mostly. A fast SSD on a fast I/O connection will do a several hundred thousand to more than a million operations a second. Here, you don’t have a fast hard disk. And with little fast storage memory (RAM) as is the case with these Intel 8 GB iMacs, your apps and caches are either very small or are entirely unavailable, which means that you’re necessarily accessing that hard disk even more often, moving parts of macOS and parts of apps in and out of what little main memory is available.