I think this problem occurs with AMD RADEON video cards after testing Affinity Photo on 3 different systems,
First system: Windows 10 home intel core i7 6700k cpu 4gz , memory 24,000 mb video card NVIDIA GEFORCE 750 TI memory 2,048 mb NO PROBLEM
Second system :Windows 7 Pro Intel Xeon 3360 2.8 gz memory 8,000 mb video card NVIDIA GFORCE GT 220 memory 1,024 mb , a little bit slower but NO REAL PROBLEM
Third system : windows 7 Pro intel core i7 3770 3,4 gz memory 16,000 mb video card AMD Radeon HD 7800 series memory 2,048 mb Very very slow and crash!!