The point is that the tile size should be left at default 2048 for most cases. Tile rendering was removed at one point because progressive refined rendering has been sped up enough in Cycles X that it was on par with tiled rendering so tiles are not needed for speed anymore. But they found out that when rendering high resolution, the high res buffer would be too memory-heavy and crash blender so they brought back tiled rendering just for that. They set the default size to be 2k so that tiles will only be used in higher resolution rendering. It should not be adjusted for speed now, since speed was not the purpose of tiled rendering anymore.
I think it’s a bad idea to use noise threshold 0 (automatic), since you just lost your control over the quality of the image this way. The ideal workflow now has changed from sample-based to noise-based.
So basically you would use the default max sample 4096 most of the time, and use the noise threshold to adjust the quality you want to have. Cycles will automatically detect whether each part of your image gets to the noise level you set, it will stop sampling the part that has reached the noise level, and continue sample parts with more noise. So now the workflow would be to use 4096 as max sample most of the time, and use a noise threshold of your liking to determine the quality of your final render. The smaller the threshold the better the quality. Default 0.01 is already quite good, and I usually just use 0.05 which is not as good quality but faster.
It has also been discussed in another topic but I don’t think branched tracing would come back. Here was the thread: