This issue was alreday reported in 2017 and 2018 for V 17 of Kodi.
Using the alpha version of LE and Kodi 18.3 on a Raspberry PI 4 I see the same issue:
Although the duration of showing a picture is set to 3 sec, it's displayed a lot longer. Also zooming and sliding the pictures in a slideshow is VERY slow.
Trying the same Kodi version on Amazon FireTV stick or on a Windows computer works fine.
Can anyone confirm that?
If needed I'll create a debug log.
Thanks Uli