I7-8700 with 32GB RAM and a lower-end video card than yours (GTX-1060).
Don't know the answer to your query, but something is amiss if it takes two-minutes to import a 1GB file. Close other programs while editing.
Testing again, this time in v8.68 beta, a 1.5GB mp4 file launched VP and displayed in preview in less than twenty-five seconds.