If you have performance problem on Cisco 1721, Cisco 2610XM-2651XM, Cisco 2691, and Cisco 3660, with WIC-1DSL – IOS 12.3(2) or above, probably depends on the default value associated with ALL5 clockrate.
If yuo wanna check type the following command on your router:
hellroute01#show controller atm0/0 | include ATM0/0
Interface: ATM0/0, Hardware: DSLSAR (with Alcatel ADSL Module), State: up
SCC0 = 2600000 (ATM0/0)
SCC3 = 1000000 (ATM0/0)
If you get 2600000 (default value) on SCC0 or SCC1 your downlink speed rate probably is limitated at about 300 KB/s
For get full speed, put max supported value on aal5 clockrate like this:
hellroute01(config-if)#clock rate aal5 ?
<1000000-7000000> clock rates in bits per second, choose one from above
hellroute01(config-if)#clock rate aal5 7000000
Becareful the atm interface is automatically restart for apply the change. Then you lost connection on this interface.