Just speculation on my part, but it probably has to to with generating thumbnails, which can take some time if you have a project that has lots of events. I used to have thumbnails for head and tail, but switched to head only which seemed to speed things up considerably. Since my system has a relatively weak video card (Nvidia 650), I’ve found that waiting till that processing is finished really helps preview performance, especially for 2.7K or 4K video. But again, I may be completely off the mark.
wwaag