I've come across this problem running a batch job while following the Affinity Photo Workbook. I was doing the Chameleon exercise and the batch processing always made the programme hang. As suggested, I unchecked Parallel processing and it worked okay. However, I have 24Gb of RAM and 500Gb free space on my hard drive so I would imagine it should be able to process the files simultaneously. Sorry. Should've said I'm running macOS Mojave 10.14.3 on a 27" late2012 iMac 2.9GHz Intel core i5.