First and foremost, presenting Premiere with a variety of frame sizes, rates and codecs is a guaranteed way to slow the entire editing process.
Sure, Premiere CAN transcode on the fly, and play back whatever you dump into the timeline…but imagine that Premiere is the United Nations, and your project is a huge meeting about some important worldwide topic, like world peace, and that meeting is being attended by negotiators from all around the world.
Do you think discussions would proceed a bit more quickly if everyone was speaking a different language, or if they all agreed to speak, say, French? That’s why we have the term “lingua franca”…a common manner or speaking for all involved, so that communication is facilitated.
Similarly, best practices in Premiere dictate that, if you have a variety of codecs and frame sizes, you transcode into a common “language” for best performance. If that seems like a timewaster to you, consider:
1) how much time you’re wasting waiting for pending media, and;
2) how much time you’re wasting waiting for Premiere to transcode on the fly so you can watch.
With the ease of invoking Media Encoder, and the ease of setting up transcode presets, this would be the first thing I’d do to get better performance.