Exceedingly laggy playback despite 720p30 video, 0 effects of any description except basic keyframes

As described in title.
I am editing 3 clips together in rapid succession that are 720p30, with no effects except some basic scale keyframes.
Premiere refuses to play them at an even remotely acceptable standard, even though global fx mute is on and the video is set to 1/4th preview quality.

There are other, more complex sequences in the same project, with multiple tracks of video playing at once, alongside many blending modes and grading effects. This it can play just fine, but this specific sequence that is simpler in every concievable way explodes it and makes editing impossible.

Rendering the sequence in-to-out takes a stupendously long time for a 10 second segment consisting entirely of low-quality, low-framerate video with no effects. (see attachment)

The problem is almost certainly not my computer, as I can easily play more complex sequences in the same project (see earlier.) Additionally, Premiere never uses over 40-50% of my CPU, and barely utilises more than 10GB of the 27GB of RAM it has allocated. At most, it will use about 5% of my GPU.