IE Plugin does indeed require administrative rights as it has to register some OCX components in Windows. There is no other plugin similar to this, except for using Desktop instead, which I'm afraid it's not a solution as long as it requires permissions as well. This all is documented in the Reference Manuals.
The logic option would be to use the Ajax client and find out why is degrading performance, when it is supposed to do the opposite. Moreover, the Ajax client has been particularly enhanced in the previous 11.20 SR2 release 11922 (current is 11.20 SR3 release 12018). I'd suggest you to upgrade to the latest version available and log a Support case if it keeps behaving so poor.
Hope that helps.