As Ann suggests, trancode or create proxies on ingest. For PC, try one of the DNxHD/HR codecs. You can also check out ProRes. I like ProRes LT, but I am on Mac.
The cool thing about transcoding or proxies on ingest is that you can start working and the clips are automatcally added and linked to your current edit. It's way cool.
Or is there a point where processing the code becomes the problem, not HD speed.
Not the "code" but the codec - the compressing and decompressing of highly compressed formats like Sony's XAVC takes not only drive speed but CPU theads, RAM, and GPU working in concert, especially at 4K. Editing codecs are not as compressed and, therefore, have that "buttery" playback experience you are looking for. I just transcode on ingest to ProRes LT and I have just that. It's nice!
If you don't need 4K and only HD, you can have even better performance as you can scale the footage at ingest, saving you those computer cycles during the edit that were formerly being used to display 4K.
Personally, I transcode to ProRes LT on ingest and use smart rendering for export. Works great, exports are lightning fast and very reliable, and no proxies. I delete the LT files after the project is complete.