H.264 slow scrubbing and playback performance
Usually in these situations I'd say maybe you have a bottleneck from your storage media. But with those bit rates any Ssd or hdd should be fine unless the files are really large. Thus it sounds to me that you have either a bottleneck with your GPU, Cpu, or Ram. All are equally important for smooth playback. I'm guessing the issue is the increased resolution of the second clip.

Without knowing more about your specs I would say your best options are to create low res Proxies. In premiere it's pretty easy, you just have to modify your project ingest settings. You might as well create Proxies for all the files, it won't matter anyways as it will relink on export.