In your previous post you gave media data from one of your video clips that shows the files are H265 (HEVC) and showing a wildly variable frame rate that could be responsible for most if not all of your problems. If possible you should be working with files that have a constant frame rate which is much easier for a video editing program to work with and needs much less processing power. While such files use up less disc space they demand a lot more calculation in use for loading, playback and processing. You may be able to reduce loading times by selecting modes of viewing that require less processing such as selecting the option within the program to only show the first and last frames of a clip in the timeline but don't expect much improvement.
If you only have one monitor and no additional inputs as John suggests the only way to activate it would be to buy a headless ghost of the appropriate type (HDMI should do it) to plug into the motherboards GPU socket. That should activate the GPU for use with a video editing program. They are relatively cheap to buy and easy to install.
Just plugging one monitor into the output for the Intel GPU at the back of the computer should be enough unless you use both screens for gaming and need the possible higher refresh rates.
There should be no reason to have to go into the EUFI bios where you would still have to either be using a monitor in the Intel output socket or a headless ghost. The GPU must think it is powering something.