Sorry, I don't have any direct experience with your problem.
It appears CEF is supported in your IOS version. What happens when you enter 'ip cef' in config mode [ router-1720#(config) ip cef ]? Does it show up in the running config? Do you get anything when entering the command 'sh ip cef summary' in enable mode?
Be aware the 12.1(T) IOS train is no longer updated and is subject to multiple vulnerabilities, so an upgrade is recommended.
If you enable CEF and then create an access list that uses the log keyword, the packets that match the access list are not CEF switched. They are fast switched. Logging disables CEF.
When performing a 'speed test', what is the destination IP, and is the routing distance/cost from the 1720 to the destination IP the same over both connections?
From the link shown below, it appears you should issue the 'no ip route-cache' command on the ethernet interface of the 1720. Note the warning that this may crash your router.
You may also need to use these commands: 'no ip load-sharing per-destination' and 'ip load-sharing per-packet' on applicable interfaces.
If you use a VPN connection across the T1's and have the VPN daughterboard in the 1720, you may suffer severe performance loss if you enable CEF on a newer IOS version.