Is my HDD throughput normal? I am a musician, and was finding my 2011 iMac was getting sluggish with more complex tracks. It was topping out the CPU so I bought a second hand 2015 iMac with a much more powerful CPU. I am finding that the new machine is actually considerably worse in performance than the older one. Activity monitor reveals that there is plenty of CPU time and physical memory available. The problem seems to occur whenever I try to do more than one thing at a time which requires any amount of disk access. As I type this, my Dropbox has just started syncing, and the performance of the machine has degraded to the extent that I am typing, and the letters are appearing slowly one at a time on the page. If I try to open a piece of software while another one is doing anything, the whole system grinds to a halt, I get a persistent beachball, and software will start to freeze and/or crash entirely. Despite this, Activity Monitor continues to assert that there is no problem CPU or memory-wise. So I did a disk benchmark and was surprised at how low the results were: 



Dragging and dropping that image into Safari from my Desktop, by the way, required me to wait for Exposé to redraw (and it stuttered rather than swishing smoothly), click and drag the file, un-Exposé and wait again, drag the file into Safari, then wait for the beachball to stop before letting go. From experience, if I drag and drop without waiting for the beachball to stop, the Mac ignores the drag and drop and I have to do it again.

Opening my Applications directory can take several minutes. Opening music project files? I go and make tea while it thinks about it. It is so bad that I have actually put the older 2011 iMac back into the studio as this new one is completely unusable. It has been like this since I got it. When I bought it I erased the HDD and installed Mojave from scratch. It has always been this slow—it has not degraded over time.

This is clearly not the expected performance for this machine, so if the CPU and memory aren't causing the problem, could it be that the hard drive is either so slow as to bottleneck the performance, or is failing?

Hardware Overview:

  Model Name:   iMac
  Model Identifier: iMac16,2
  Processor Name:   Intel Core i5
  Processor Speed:  2.8 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: 229.0.0.0.0
  SMC Version (system): 2.32f20
  Serial Number (system):   C02SF1JTGG77
  Hardware UUID:    C3F0A7B9-1498-5EA2-94D2-B1CBE28C6E50

APPLE HDD HTS541010A9E662:

  Capacity: 1 TB (1,000,204,886,016 bytes)
  Model:    APPLE HDD HTS541010A9E662               
  Revision: JA0AB5D0
  Serial Number:    JD8002D81S8JRD
  Native Command Queuing:   Yes
  Queue Depth:  32
  Removable Media:  No
  Detachable Drive: No
  BSD Name: disk0
  Rotational Rate:  5400
  Medium Type:  Rotational
  Partition Map Type:   GPT (GUID Partition Table)
  SMART status: Verified
  Volumes:
EFI:
  Capacity: 209.7 MB (209,715,200 bytes)
  File System:  MS-DOS FAT32
  BSD Name: disk0s1
  Content:  EFI
  Volume UUID:  0E239BC6-F960-3107-89CF-1C97F78BB46B
disk0s2:
  Capacity: 1 TB (999,995,129,856 bytes)
  BSD Name: disk0s2
  Content:  Apple_APFS
I am experiencing something similar "upgrading" from a 2011 MacBook Air with SSD to a 2017 iMac 4K with 1TB 5400 disk (same disk as yours). 

The read and write speeds looks what you could expect from such drive. 

You could try reboot into safe mode to see if there is any difference. 

Another solution is to buy an external SSD to hook up via USB/Thunderbolt for your project files.