iMac runs incredibly slow on startup and when first opening applications

I have a Late 2015 iMac running on Sierra that has been incredibly slow since the day I got it (May 2016). On original setup, I used time machine to transfer everything over from my work computer since this would be my work from home computer. When first logging in, it takes forever to load the desktop, files, open applications. Once I open an application once, it usually speeds up when opening a second time. I took it to the Apple store and they ran diagnostics and said everything was normal. I know that taking over a minute to open Firefox is not normal.

I was reading another closed thread New iMac 2016 so slow.. what is going on? that sounded very similar to mine so I ran the EtreCheck but I don't know how to analyze the results.

macOS Sierra

Version 10.12.6

iMac Retina 4k, 21.5 inch, late 2015

Processor 3.1GHz Intel Core i5

Memory 8 GB 1867 MHz DDR3

Startup Disk Macintoch HD

Graphics Intel Iris Pro Graphics 6200 1536 MB

Model Name: iMac

Model Identifier: iMac16,2

Processor Name: Intel Core i5

Processor Speed: 3.1 GHz

Number of Processors: 1

Total Number of Cores: 4

L2 Cache (per Core): 256 KB

L3 Cache: 4 MB

Memory: 8 GB

Boot ROM Version: IM162.0207.B20

SMC Version (system): 2.32f20

Serial Number (system): C*************8

Hardware UUID: 41864F5A-6517-552A-8860-00DB67EC4154