this is 4k video you need to work with proxy file that means activate proxy. Your computer has not the power to handle the original file to work you need to transform the original movie in file lower definition that call proxy.
As mentioned, you are working in 4k NTSC and it is quite likely that your computer is under powered for that level of footage. It's difficult to advise without more information on your computer spec, processor, ram, windows version etc.