What is the exact number of photos whose count you give as "hundreds of photos" and what are the pixel dimensions of those photos.
Photos oversized for the project will not make the end product quality better but rather results in project failure at the level of either (a) Photoshop Elements Slideshow Editor (b) in the transfer of the Photoshop Elements slideshow (c) Premiere Elements editing and export.
So, lots of questions that need answers
a. How many photos and what pixel dimensions (photo width and height in pixels)
b. What versions of Photoshop Elements and Premiere Elements are you using?
c. What Output option are you selecting for the Photoshop Elements Slideshow Editor slideshow and what project preset are you or the program setting for Premiere Elements (you can check Premiere Elements project settings Edit Menu/Project Settings).
I just did a mini test run in Photoshop Elements 11 Premiere Elements 11 where
a. Photoshop Elements slideshow included Fit Slides to Audio resulting in slide duration of 33.9 seconds
b. After Output (Edit with Premiere Elements Editor) and Break Apart Elements Organizer Slideshow, the duration of each "slide" was 33.27 seconds.
So I am not confirming your results.
The answers will be in the details that you present.
Please review the above and then we can decide what next if necessary. As an aside (getting ahead of the story), do your computer resources support your project with hundreds of photos and 15 sound tracks et al? How much computer available RAM and free hard drive space?