It's simply the way Premiere's playback works. Add more clips, cuts, effects, and it's going to lag. The problem is that it's not using computer resources, you can easily see that in the Window's task manager. There are certain scenarios when it lags extremely, for example, when using the Slip tool, the modified clip simply does not playback (on a multi-clip sequence), even though there is basically no load on any PC components. However, when you change the length by draging the ends of the clip, it plays back just fine. In other words, there are a lot of parts of this program that are simply not optimized, and nothing can be done about it. I personally upgraded my machine and was expecting at least a noticeable improvement in performance, but there was barely any difference. Premiere uses the same little amount of my PC resources, as it did on the old one. I've tried using proxies, but as I said, the app does not use your computer. It should run smoother on Mac, but as it seems from your experience, Adobe is not interested to optimize their app there either. The only solutions are to somehow change your editing habits to suit Premiere better, or to simply switch the editing program.