A Impress converts included jpg images to png.
1)I start with slide show including two slides. 
2)On each slide insert 3 jpg images (size of images 1.5 Mb).
3)On the first slide add animation (on click) to the images on the first slide.
4)Save this file check the unzip the saved file check that the images are in jpg
format.
5)Change the presentation order of the images in the first slide 
6) Save this in a new file and check the unzipped odp still jpg images.
7) Start and run the presentation
8) Save the presentation in an new file unzip and at least one jpg file is
converted to png. This save operaton takes much longer time.

9) Create a big presentation whith a lot of images chnge animation orders for
the images.
10) Run this presentation for a customer you try to impress ;( 

Comment 1:
In presentations including 10 slides with 2-3 images on each slide. Will result
in very big files and takes looooong time to save. This presentation has lousy
performance.
Comment 2:
There are a lot of chat about lousy performance and big files in the forum.
Mayby this is one of the reasons.