Hi Michael and thanks again for your detailed reply. Much appreciated!

> "I have seen that just the general switch from x87 floating point code to x64 SSE floating point code seems to give a speed boost"

This is a very good thing...

> "It is actually possible in some cases for conversion to 64-bit to actually reduce performance due to doubling the size of data that's being moved around but it doesn't seem like that is going to be the case here."

This is an even better thing! A sigh of relief....
It's true. Especially for application that handle big chunk of data, the "upgrade" to 64bit often reduces the performances, but this is almost true only
for low-level hardware (poor cpu and old video card, and outdated ram/pci bus).

> "The only thing I'm a little worried about is the 64-bit QtWebKit doesn't have as high of a JavaScript JIT engine enabled for it, I'm not sure yet if that will impact script performance very much or not."

I don't know anything about this area of programming but I read this article : https://wiki.qt.io/New_Features_in_Qt_5.1, dated 25 March 2016.
If you search "Qt WebKit" into that page you will find this annotation :

"Qt WebKit :
JavaScript JIT on Windows 64 bit."

What do you think about ?

Best wishes for all your job and good night. (again)