Right, I've asked myself this question many times.
Why does the keyboard scrolling never reach even half the speed of normal mouse scrolling, and why does it delay for so long? It was worse in IWD1/HoW, but still...
Would this be because it was based off the BG1 engine? I would like to know if there is a way to fix it in both IWD2 and IWD1.

You can adjust keyboard and mouse scrolling speeds to your taste via options.

If you still think it is too slow, well I don't know any fix about it, sorry!

It's not slow. It stutters. Just like in BG1 and IWD. Apparently BG2 is the only IE game in which I've encountered smooth keyboard scrolling.