Why is my MacBook Air faster than my iMac? I'm wondering why my MacBook Air (13 inch, early 2015) is so much faster than my iMac (Retina 4K, 21.5-inch, Late 2015)?

The MacBook Air is configured with a 1.6 Ghz dual-core i5, 4 GB RAM, Intel HD Graphics 6000 and a 128 GB SSD.

The iMac is configured with a 3.1 Ghz quad-core i5, 8 GB RAM, Intel Iris Pro Graphics 6200 and a 1 TB SATA hard drive.

I run similar programs on both, some of which you would think would slow down the laptop, yet the laptop always seems to outperform the iMac. The iMac has been incredibly slow lately with Safari being especially painful at times. I thought maybe it was b/c some of my programs probably require a better machine than I have but then those same programs perform great/much faster on the laptop (which I'm sure does not have the min. specs for them). 

The iMac is incredibly slow especially with opening just about everything from Mail, Photoshop to Safari (yet Firefox opens right up). The heavier program I run on both my machines is called Archicad v. 23. Archicad is much faster to open on the laptop. 

I'm adding a link to some screenshots from the laptop and iMac to compare and screenshots of the current Activity Monitor from the iMac which I just restarted (it would only let me upload two images here so I added the link instead).

https://ibb.co/album/fMBk6F

UPDATE 3/14: These screenshots here are from the iMac - not sure which are best to post i.e. CPU or Memory, etc. it only allows two images. Also should point out the iMac can run a little better once everything has been loaded and it's been sitting for an hour or two...but the start of programs and getting it running at normal speed after a restart takes a long time.
One main difference between the two computers is that your MacBook Air has an SSD - a solid state drive - for data storage, whereas the iMac has an ordinary hard drive. SSDs are typically an order of magnitude faster than hard drives, so if your usage of the computer involves a lot of disk usage, that could explain the perceived speed difference.

Another difference is the processor. Here the iMac has the advantage with a CPU that potentially is up to about 3 times faster than the CPU in the MacBook Air. However, if your specific usage of the computer does not involve a lot of heavy CPU usage, or your programs are not written to take advantage of multiple CPU cores, then that advantage could possible be minor.

Even considering these differences between the two computers, you shouldn't have a "painfully slow" experience on the iMac. If it was quick, and now is painfully slow, it doesn't sound like the hardware is just insufficient for the task. It sounds more like a software problem or possible a hardware defect.

I would suggest testing the software hypothesis by installing macOS on an external USB drive and see if that improves performance - or if you have no problem rolling your computer back to an earlier date, then by restoring an earlier Time Machine backup from before you've experienced the slow downs.

In terms of a hardware defect, you could suspect something like a cooling problem. If that were the case, you would probably hear increased fan noise, and/or you would see kernel_task as having a high CPU percentage in Activity Monitor, which the screenshots does not show. Therefore it doesn't seem that a hardware problem is likely just now. You could check your logs (using Console.app) to see if you have any error messages about your disk drive - timeouts here could give massive slow downs throughout the system.