I am composing a movie of primarily still images and a few movie clips. So in the time line I have 6 still images and 3 movies clips (non 4k) for a total running time (becasue I can't get past the sixth clip) of 1.06 seconds - yes I said ONE SECOND. Seeing 6th clip as the 5th clip is one issue. It is very sluggish when I try to change the image size, to the point that I can go for a walk. It locks up during playback and freezes if I hit render in to out. I have to turn my computer off, can't even do a force quit. Everyone in the house is having trouble with premier products with the lates update. I have tried the previous version and I am having the same issues.
I have watched all of Colin Smith on playback, stuttering, and cache issues. I have addressed all of those.
I would check to see what format your stills are in (Jpeg?) and how big they are (pixel dimensions).
If the images are large - try resizing them to something smaller in Photoshop. Start by resizing them to approx the same pixel width of your sequence settings (I would normally go slightly bigger to allow some image scaling inside of Premiere Pro if I wanted to animate the photos ... but let's see if this works first).
Also save these resized images out of Photoshop in a less compressed format like .png or .tif