Is my HDD throughput normal?
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.