Hardware is only one side of the coin when editing. The other side is the media, and I think you're underestimating the effect it has. The type of codec you edit with has a massive impact on your editing performance (yes, even on a good computer). You're editing 4k, high framerate footage that's likely in an interframe codec like h264 or h265. I'm not surprised at all that you'd have playback issues. That's normal. Create proxies into an intermediate codec and work away.
Professional editors work with professional editing codecs-and if your media source isn't an intraframe editing codec to begin with, then you want to make it one.