iMac 2013 - SSD Upgrade - Poor Performance

Hi,

I have upgraded the internal hard drive on my iMac with an SSD (Crucial MX500 - compatible drive).

I installed a clean version of High Sierra, drive formatted APFS.

First issue - the internal fan spun at maximum RPM. I installed Macs Fan Control to control it.

Second issue - the performance of the iMac is now very slow.

So, I installed a clean version of Sierra (drive formatted HFS+) and had identical issues.

I've enabled/disabled TRIM, tested the SSD with various software (all positive), reset SMC/NVRAM.

I've attached an Etre System report and frame grab of the Activity Monitor.

Is there something I'm not addressing?

Thanks in advance,

Simon,

iMac 27 inch, late 2013

3.4GHz i5

16GB 1600 MHz DDR3

NVIDIA GeForce GTX 775M 2 GB