Have you recently added any plugins to Premiere? I've found that plugins can really slow down performance for me. (Like Frame.io or REDGiant)
Another thing you could try is to the global effects mute during playback. Especially with 4K video files.
Click the plus symbol in your program window to add the icon for the "global effects Mute".
It looks like an "fx" icon.
I feel you but it is slow even when colour grading a jpeg photo I imported ♀️
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It's not just the amount of RAM that your MacBook Pro has, but It's a combination of three other things. First, your system's CPU is too weak for any type of 4k video work whatsoever, largely because it is not a true quad-core CPU at all but merely a hyperthreadable dual-core CPU. (All mobile i5 CPUs up to and including the Broadwell generation have only two physical cores.) Second, you failed to indicate whether or not your system has a discrete GPU or is merely using the integrated Intel graphics (I assume the latter). Third, you are trying to run everything off of that single molasses-slow hard drive (which I am assuming that it only spins at 5400 RPM).
And to top that all off, newer versions of Premiere Pro really needs a much more robust hardware configuration than past versions just to perform adequately smoothly.
Premiere CC 2018 just sucks, did you try CC 2017...I know it's still made by Adobe (garbage software made by idiots) but somehow they didn't messed CC 2017 up enough, up to their standards (which they have none) and it got released somehow, i recommend that version.
This why I always use proxies, convert any 4K to 720p while editing but like I say I’m still having the issue no matter how small a file I use and it is slowing my Mac down even when it’s not open -_-, I feel going back to 2017 cc may be my only hope guys !!