I don't see why it wouold be slower. In everything I read x64 was supposed to run 32 bit programs just as fast as normal and in some cases there were actually 1-2% performance increases on 32 bit programs! It runs blazing fast for me, Internet Explorer has never been soo speedy. Well I didn't try farcry, but every game I have tried so far works flawlessly on 64 bit windows.
Perhaps check the installation folder when it is installing it. It should be installing itself to Program Files(x86), not Program Files.