In practice I've found the performance savings often goes the other way--for large (multi-GB) images it's faster to split it up into more layers that it can download in parallel from the registry. It won't parallelize the download of a single layer and in EC2+ECR you won't get particularly good throughput with a single layer.

Depends. If you would have to fetch a big layer often because of updates, that's not good. But if what is changing frequently is in a smaller layer, it will be more favorable