How can I speed up my new (but slow) 2017 iMac?
Why does it feel so slow?

Your iMac has a 1TB 5400rpm SATA Hard Drive and this is what’s causing the bottleneck. Your mid-2010 iMac would have had a 7200rpm Hard Drive. To fix this you really need to address this bottleneck.

The 'easiest' solution

Since your iMac has two Thunderbolt 3 (USB-C) ports that support speeds up to 40 Gbps (via Thunderbolt) and 10 Gbps (via USB 3.1 Gen 2), the easiest solution is to use an external SSD that’s connected via one of these ports and to install macOS on that and use that drive as your main boot volume. However, you need to ensure that:


it’s definitely an external SSD (and not just a hard drive)
it supports either Thunderbolt or USB-C (i.e. USB 3.1) as its interface


Both of the above are critical if you’re wanting to improve speed. However, you’ll probably find Thunderbolt models are too expensive, so opt for USB-C / USB 3.1. Do not get confused by USB 3 - this is not the same as USB 3.1. And remember, it needs to connect to one of the two Thunderbolt ports (not one of the USB ports).

Some examples of the types of external drives I mean are:


Samsung T5
SanDisk Extreme 510
G-Technology G-DRIVE mobile SSD


Once you have the drive you want, you will need to install macOS onto it and migrate your data. 

NOTES:


You don’t necessarily need to buy a 1TB model (although this would be ideal). The main thing is that you have enough space for your macOS installation and all your apps.
Once set up and running okay, you can use the internal hard drive as storage. If you opt for a 500GB external SSD instead of a 1TB one, you may want to consider having your Photos and/or iTunes libraries stored on your internal hard drive instead. Maybe also your iOS backups.
To install macOS you will need to reformat the drive so that it’s formatted as a OS X Extended (Journaled) drive and that its 'scheme' is GUID Partition Map.
While this iMac is not officially upgradable after purchase, it can actually be done. I upgraded the same model before Christmas so that it had an internal SSD for storage and a lot more RAM. However, I’m guessing this isn’t something you’d want to tackle (I wouldn’t recommend it unless you’re very comfortable with the idea of doing it), but if you are I’m more than happy to chat to you about it.


Let me know if you have any questions or would like to chat about doing an internal upgrade.



Case study

Today (24 Jan 2018) I installed macOS Sierra 10.12.6 on an external SSD (specifically a 500GB Samsung T5) and did some comparison tests to try and provide an indicative idea of the type of speed improvements you could expect.

The tests were performed on the same model iMac as yours (i.e. mid-2017 Retina 4k iMac with 8GB RAM, a 3.0 GHz quad-core i5 CPU and a 5400rpm SATA hard drive). Obviously, your results will vary depending on the software/login items/fonts/kernel extensions you have installed.

Below are the results of my testing. 

Test 1: Boot up time (to Login Window): 

  Original boot up time: 1m 23s

  New boot up time: 0m 35s




Test 2: Login time (to full Desktop): 

  Original login time: 1m 15s

  New login time: 0m 9s




Test 3: Launch MS Excel:

  Original load time: 0m 8s

  New load time: 0m 3s




Test 4: Launch MS PowerPoint:

  Original load time: 0m 6s

  New load time: 0m 3s




Test 5:Launch MS Word:

  Original load time: 0m 14s

  New load time: 0m 4s




Test 6: Launch Photos app with a 380 GB Photos library:

  Original load time: 1m 23s

  New load time: 0m 39s




Testing notes:

1. macOS and all apps were installed on the external SSD.

2. All tests performed three times and the results averaged. 

3. Each time tests were run, they were run after a fresh reboot. 

4. For Test 6, although the Photos app was installed on the external SSD, the 380GB Photos library was stored on the internal 5400rpm SATA hard drive.