I got curious and reinstalled Animatics 2.1.6, to run some quick comparisons.

When it comes to browser behavior, the album is more smoothly presented in Firefox. In my tests, no real difference in Brave or Chrome.

The other big difference is the use of screen real estate. The older version is more of a "full screen" presentation, truly filling the viewport at all times. That's fine if most of your images are landscape mode, but portrait mode images are severely cropped, which some users find objectionable. And even the landscape mode images are often cropped, though it isn't quite as obvious.

But you can achieve the same thing in the latest version by choosing Animatics > Lightbox > Main image > Scaling: fill.

So, the one sticking point is the poor performance in Firefox. I'll have to leave that to Laza.

ETA: I've updated my demo album to use the "fill" mode.

The Ken Burns effect is not for you if you'd like to display the pictures without cropping.

This effect uses zoom-in and out in a random manner. While zooming out can result in a 100% fitted image, zooming in produces a picture that's larger than the viewport at the end of the animation. This transition was designed for videos, which are typically in a landscape format.

The smoothness of the animation is affected by the video card. The newer versions attempt to offload more of the animation to the graphics card, which, in a poorly optimized environment, could be even more choppy than the CPU processing. Perhaps Firefox is the culprit here. I rarely test that browser, as that's down to 2% these days. I will check the smoothness for the next version.

But I'm seeing smooth animation with 2.1.6, but choppy animation with 4.0. Same machine, same video card. Something else was changed "under the hood." If I get time, I'll start experimenting with Animatics versions to see if I can pinpoint when the change happened. Lots of other stuff going on at the moment (for you, too).

What was changed is only the use of requestAnimationFrame. I don't remember the details, but starting multiple animations in the same frame allows the browser to offload these animations to the 3D video card, so it is generally a better way of doing this. However, sometimes it even gets worse, as the handover requires time and resources. And this changes with every browser and video card combination. So, A) it's impossible to test (no debugging possible with animation frames), B) yields different results on different PCs.

Given all of this, I would advise the original poster to set the JPEG Quality to 85%, at most. Going higher yields much larger files, but doesn't produce any visible improvement. In fact, in a double-blind test, it's probably impossible to see the difference between 75% and any higher setting.

I would turn off HiDPI images. This yields massive files, but again, in a double-blind test, it's virtually impossible to tell the difference.

I don't think the original poster is using variants, but in a skin like Animatics, in which the display size of the image might be changing while you're viewing it, using variants seems like a recipe for trouble. I'd be inclined to disable them in the skin.

Image bounds of 1920x1080, 85% Quality, no HiDPI images, no Variants.

(On the other hand, HiDPI thubmnails make a noticeable difference, at a very small cost in file sizes. I actually force that setting in my skins, since they're often doing things like masonry layouts and "expand on hover" effects with the thumbnails. Having those extra pixels makes a big difference.)