I have been handed a tiny file 3.7MB which is almost impossible to open using Power BI Desktop (now on 12 October build). If I dial my VM up to 32GB and 4 Cores I can just open it after about 10-15 minutes, with it using 100% CPU and 98% RAM. Until the end of that process, it just shows the splash pane, not the main Power BI window.
It turned out the issue was the Background preview refresh option (on by default). Once that was unchecked, the file behaved as normal - memory use was less than 2GB.
It's so bizarre - never seen anything like it and I've worked with files from multiple clients every day for 6 years.