You made me check. Since August 2018, I've cut about 1d using variable speed, and 1.5d using silence trimming. I'm only using mild at the moment, and have used medium before, but mad max was too much for my taste. But since I've listened for 83d, silence trimming is a <2% saving for me, so it might not mean much.
Edit: I do use it extensively specifically when catching up on the backlog of a podcast. So it probably accounts for way more during those times.
I would think podcasts without RSS is a tiny minority, and I wouldn't take them seriously. And I've never even heard of ivoox.