What will result in highest performance for 32-bit App - "Win32 on 64 PC" or "Win64 on 64 PC"?
The answer is - it depends. The only way to find out is to perform benchmarks.
Sometimes 32-bit applications run faster, sometimes they run slower.
There are a lot of competing factors affecting the performance.
Generally the difference in either direction isn't very large.
I don't see how you can legitimately exclude any factor from
performance considerations, because performance always happens in the
real world where all factors are in play. Speed is not theoretical and
cannot be treated in isolation.
Here is an article where the author did some benchmarking:
He found that the 32-bit binaries were faster in his tests. That was
on SPARC in early 2004.
Here is a 2010 article from Intel about moving to 64-bit applications:
According to this article, 64-bit binaries can giver you faster math,
so math-heavy code should be faster if correctly optimized. Otherwise,
your 64-bit code may well be slower, particular since it takes more
memory and this may result in more time spent accessing RAM (more
frequent cache misses).
Source Do 32bit apps run faster or slower on a 64bit OS? [closed], answer by nate-c-k