I had similar issues on my slightly old 4K gaming PC (i7-4790, SSD, 16GB, Win7, GTX1070). I was trying to edit a four-camera shoot of about an hour of mixed 1080p, 2k7 and 4k footage and it was nightmarishly slow. I tried all the cache, preview size and so on suggestions with minimal improvements.
Turned out that Windows had 'helpfully' set the Power Plan to 'balanced', which meant the 3.6/4GHz processor never went faster than 800MHz; changing the Power Plan to 'High performance' made it work at the sort of speed I'd expect.