Assuming P Elements has the same 'time altering' engione as Premiere Pro then this is because when you slow the thing down to 50% the render engine simply plays each frame twice. This will cause the thing to look jerky.

You may find there is a frame blend mode if you are lucky. This will blend one frame into the next over the designated interval. Better than simply playing each frame multiple times but nowhere near good enough for what we all really want. The blend mode works better when speeding clips up than slowing them down.

What you need is a plug-in like Twixtor to do 'real' slo-mo. Have a play. I think you'll be impressed.

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