Well slideshow can me 2 things... an OBS internallly managed slideshow... in which case you use OBS to control the slideshow
OR
You are using another application (PowerPoint, Windows Photo, etc) to create/run the slideshow, and you are capturing that application screen (Window, Display, etc) with OBS. in this case, you control the slideshow with the slideshow app (whatever that happens to be)
*IF* you are using OBS's internal/native slideshow capability, and the timing is arbitrary, I'd suspect you either A. chose a random interval, or B. your CPU is overloaded and can't keep up enough to be timing consistent (in which case, you'll probably be having plenty of other issues as well)