Thanks for the suggestions, this resolved my issue which was down to the number of threads. 'Tools', 'Preferences', select the checkbox 'All' at the bottom next to 'Show Settings', 'Input/Codecs', 'Video Codecs' and 'FFMPeg' under 'Threads' changed it from 0 to 4 (I suspect 0 is use all cores?), 'Sa...