How do you do the rendering? Is it sorted (radix?) instances? Do you amortize the sorting over a couple frames? Or use some bin sorting? Are you happy with the performance?
On most newer devices the sorting can happen pretty much every frame with approx 1 frame latency, and runs in parallel on a Web Worker. So the sorting itself has minimal performance impact, and because of that Spark can do fully dynamic 3DGS where every splat can move independently each frame!
On some older Android devices it can be a few frames worth of latency, and in that case you could say it's amortized over a few frames. But since it all happens in parallel there's no real impact to the overall rendering performance. I expect for most devices the sorting in Spark is mostly a solved problem, especially with increasing memory bandwidth and shared CPU-GPU memory.
I've implemented a radix sort on GPU to sort the splats (every frame).. and I'm not quite happy with performance yet. A radix sort (+ prefix scan) is quite involved with lot's of dedicated hierarchical compute shaders.. I might have to get back to tune it.
I might switch to float16s as well, I'm a bit hesitant, as 1 million+ splats, may exceed the precision of halfs.