TIFF is not officially supported now and so far we can only output 8bit JPEG in html canvas through downsampling.
The downsampling is process "the thinking" part is controlled to be not hurting the user experience and it's different depending on your machine. For faster machines we can remove the thinking part but it would take up more memory (right now the footprint during processing is less than 20mb, consider this is a web app)
I agree Adobe is doing better in interpolation (the harshness) and we are working to improve that.
What OS and browser you're using? The curves and split toning works well on Mac Desktops.
We only have two developers working on this right now. Do you know where we can get lens correction profiles, is it publicly licensed?