[WingsToDiscovery]

I think you're trying to find information on something with two extremes. Most people who use these two alphabets natively generally only use one or the other.

In my case, I grew up learning English so obviously I can write lightning fast in English (especially with the advent of cursive), whereas my Japanese writing isn't all too fast except for set phrases I use all the time, and I didn't begin studying Japanese until I was around 18.

The same goes for those in Japan. Some Japanese may have an edge over English natives simply because using English in written form is generally compulsary for Japanese students, but it's still probably faster for them to write in Japanese.

The only equal way to test it would be to have someone study both from birth and then try it out.