There are many factors that may cause a slowdown, which I have no way to reproduce. I can only say that my company's Exchange server is 2003 and that it is the only non-google mail account that I have configured on my phone. I have push enabled. The hack works great with this setup, i.e. no slowdown of the phone, no extra battery drain.
If someone who is experiencing issues can post more details about the version of exchange, sync mode, and any other pop/imap mail accounts configured, I may be able to investigate the issue further.