Get a new more powerful laptop.
you need to have GPU for Mercury playback to speed performance, (you may need a better processor, I'm not sure what speed is it? how many cores? intel core m is vague to me)
below is the link for gpu mercury playback--obviously it would have been easier to have looked at that before purchasing the laptop, but now you can get one of the display cards recommended for GPU accel and install it yourself or have a local cpu store do it for you.
Recommended AMD and NVIDIA video adapters for GPU acceleration
Hope you find the answer to this--you indeed may have to spring for as cc merchant states "new more powerful laptop" if your proc is not up to par
Your Asus Zenbook UX305F has USB 3.0 ports. Since you cannot upgrade the CPU or a GPU, you could try to improve you other weakness, storage. Premiere thrives on multiple storage devices. Now you have an SSD for everthing. Try buying one of the new Samsung T1 USB 3.0 SSD's and put all your project,media, preview files on the USB drive and you may improve your performance a bit. Here is the performance I get on the my 500 GB T1:
I regularly use my Samsung T1's (I have three now) editing on my. much more powerful laptop than yours but the only thing I can think of that might help a little, and it would be very usable with any future computer that you might buy. I recently saw that a 250GB version at B&H is only $85