I know the 505 is kind of sluggish when it comes to processing data and refreshing the display screen. How fast is the 606 in comparision (i.e. how many seconds, on average)?

Most hardware, in my experience, is way slower than a computer for processing. I don't know why this is, but I suspect it's because of manufacturers cutting corners.

The 606 is generally quick for most tasks, but loading and saving large samples can be quite slow. On the other hand, it's pattern sequencer is blindingly fast to use, and has absolutely no save time at all!