Yeah I was confused by that, I'm using htop to monitor resource usage, it's reporting Hiawatha at 100% CPU usage. All the mysqld and php-fpm threads are sitting at 0%. It also appears to be any file upload, not just image files.
I had to stop and restart Hiawatha after a few minutes of uploading a 2mb zip file (WordPress Plugin), I suspect it would probably have succeeded but couldn't afford to have the live website down for that long because the whole site becomes unresponsive whilst uploading.
On the local dev site I cleared the access log, initiated an upload and took a couple of screenshots. Everything appears to complete normally except it takes over 1 minute to upload a 232kb png.
External image via https://fairalls.uk/1.PNG
External image via https://fairalls.uk/2.PNG
Bit stumped at the moment. Will try and find time to spin up a fresh Debian VM locally and a blank WordPress install and see if it's the same.