A few things here: A) Samsung doesn't make very good burners. B) Scan at 4x, not 2x. C) Ritek F1 isn't the greatest of media, it's average I'd say, bearing in mind that Ritek media has given people problems in the past couple years. You should expect to get better results than what you are seeing here though. D) Ritek F1 doesn't have good firmware support (one of the newer 16x media codes and support was added later than most media), update your burner's firmware to the latest available version. E) 16x media rarely burns as well at 16x as it does at 12x, try burning at 12x or even 8x to see what sort of results you get. I don't suggest going below 8x though, as most media doesn't burn well at more than 1 full speed below its rated speed (below 16x at 8x, 8x at 4x, etc.).
Edit: I should have read your post more thoroughly, although what I said above remains true and still applies. If you are reaching full 16x speeds properly as the create data disc test shows and yet are having slowdowns when burning an ISO image, I would look into the possibility that your hard drive is significantly fragmented and that you need to run a disk defragmenter on it. To help somewhat with this problem, in many burning programs you can increase the buffer size used, such as in Nero Burning Rom and ImgBurn. For example, try upping the buffer a significant amount such as to 128MB in ImgBurn and try burning one of the discs at 12x, and report back the results. Another thing, on my system Nero Burning Rom's buffer remains much more stable under difficult conditions such as multitasking or fragmented files, compared to other programs such as ImgBurn. Why this is, I honestly don't know.