OK, I think I've got it now, sorry it took a while. It seems there is no third party conversion software involved and my first suspicion under these circumstances is that the problem results from your camera shooting at 25 fps.
The issue is that the Photos application exports videos at 30 fps and the jerkiness you see most likely comes when your 25 fps video is converted to 30 fps. I believe you'd get much better results if your camera has a 30 fps setting and you used that.
However, jerkiness can also occur in any transitions you build into your slideshow because the photos slideshow is essentially designed to run on your computer which has much higher refresh rates than your standard TV frame rates. For this reason and especially when video is involved I recommend using iMovie to create movies, you can add video and photos and have much more control over what they do, the transitions are built for use at standard rates and you have the ability to export your movie at 25 fps which should resolve the conversion issues you are having with photos.
The photos app will create reasonable slideshows from photos and video taken on your iPhone (or similar 30 fps device), it doesn't have much scope for handling much else.