This is an example of another method besides using Render Effects, to have smooth editing with high resolution files.  We use proxy mode with Twixtor so you can dynamically change the video speed with our tools without pre-rendering previews every time, while editing.

**Note: One reason we use proxies here is that the footage is 4K and Twixtor is not a real-time effect in Premiere, so at 4K, you might have non-responsive playback even with GPU on.