I'm using PPtP and my tests have similar speeds. In fact, the VPN connection is slightly faster than the non VPN. (1.57M down/321k up on VPN versus 1.24M down/298 up on non-VPN.)
Many VPNs have set by default to send all traffic over the VPN tunnel. In my case, the figures could be the result of my remote being a DSL and the VPN being on cable modem. (Cable modem speeds in my area are much faster than most "basic" DSL connections.) That feature is usually desired for security purposes, but if you don't need it, then see if you can disable it.
And you mention that you don't have encryption enabled. Which VPN endpoint are you using? Most have encryption enabled by default and some don't have the option of disabling it. (BTW, my PPtP endpoint is on a Cisco Linksys RV042 over cable modem. My iPad is connecting to a Dlink BWLG730 802.11g access point through a Cisco PIX 501 firewall over DSL. The RV042 doesn't give me the option to disable PPtP encryption, as far as I can tell.)
(edit: BTW, the A4 CPU in the iPad & iPhone4 is basically an ARM CPU that does have much less horsepower than current model Core2 or Corei CPU. The additional overhead may be too much for the A4. But if my results are any indication, then it might not be the VPN and just a limit of the networking stack in the iOS devices.)
