As my application need to be coupled
with excel and some of it's facilities, I would have to use it right
now and right now execution time is not too bad (1-2 min).

If later aspects of application required still more computational
effort (and time increased to hours), I would have to switch to fully
programmable language. I am right now dealing with array  of 0.5
million row, 100 columns and copying data  from one row to another
etc. Later application (different) may involve more movement even
though storage requirement will not increase.

How much computational time savings could other language(like C++)
provide? (by factor of 5, 10, 20 or 100)
Is it possible to couple them well with excel allowing user to input
data in excel and see output in excel sheet? Can one write some
aspect of code in vba and one module in C++ and combine them without
much headache? I am just looking for your personal opinion in this
regard.