I've been looking into my problem for days now and I didn't find a solution to it. If there is already a question about this issue, I have not found it, my apologies. Please direct me to it if there is.
Up until now, this was working flawlessly under Raspberry Pi3B and Raspbian Jessie. My company decided to start working with Raspberry Pi 3B+, and I was told we needed to upgrade from Jessie to Stretch for them to work, but not to worry because it was backwards compatible with the regular Pi3B. And it kinda is backwards compatible, that's not the problem. The thing is that what worked flawlessly before, not even a hint of lag, is now choppy. All the animations are so slow they are more of a slideshow than a real animation.
It feels like the Raspberry doesn't have enough power to move the pictures around. It even does things like choppily play an animation on a picture that should be upside down (a non animated "transform: rotate(180deg)") but display it upright, and when the animation has finished it kinda hangs for a second, then flips the picture for a split second, then cuts into next picture/animation.
My investigation these days has pointed me towards the memory allocation option in "raspi-config", but changing that has ony made it worse. I read somewhere too, that Stretch took more ram than Jessie and there was no way around it, so I should use a "lighter" browser. I tried both Midori and Opera and while the "slideshow-effect" was less noticeable it was still there, and something in the resize algorithm in those browsers is wrong because the pictures looked horribly pixelated during the animations (they looked good if they were still, but that's beside the point).
How can I fix this? What am I missing?
I have completed a installation of Raspbian Stretch from scratch, not updating. The result is the same.
I have recorded a video (pardon my shaky hands) of what I mean by choppy. You can see that what I'm asking the raspberry to do is not that much, at all.
I hope this is a helpful information.