What resolution are the still images? When you use stills as a pan & zoom in a video application, the stills are treated as video, so if they are full res, it's like trying to edit 8K video or higher, and things grind to a halt. I've done quite a bit of this in various versions of Premiere Pro, and for 1080 presentations, I like to keep stills no larger than 2300 pixels wide. This reduces system overheads, but perhaps more importantly, prevents you from hose-piping in, out and across the frame, which looks horrible. I tend to zoom between 2300 to 1080 over five seconds.
A little tip: If you want to add a disolve between your panned and scanned frames, place each new clip on alternate timeline tracks, and overlap them by the length of the disolve, and place a disolve on both ends of both clips. This prevents the movement suddenly stopping in the disolves, which jars horribly.