I believe Simon intends to update / replace the particle system at some point (to go with the upcoming GUI) & make it a little more intuitive to use.
In regards to what you said about 1024x720... yes it's still pretty large; especially when you consider the file-size per frame.
Quick tip: when creating animations / particle type animations such as rain, snow, fog, mist, steam, smoke, things in the distant background etc, you could consider exporting them from your image / animation software at quarter of the intended size. Import them into the engine as an animation & then inside of the first frame you add a single line of Lua script to scale the animation up to the actual size you want it to be displayed on the screen. Animations such as these don't need to look crystal sharp, so a bit of blur due to scaling up isn't going to hurt your game much, but it will certainly reduce the loading time of the animations, & even more so if you convert them to webp format.
ActiveAnimations["animation name"].AnimationSize = 400 -- scale up animation by 400%