Slow ISDN transfer

[BIKEMAN]

Hi All   I configured a 2503 for a customer today using ISDN to connect. Everything is fine, DNS resolution is happening, can connect to everything BUT transfers seem to be slow - a modem like 5k ! I did a standard config, nothing fancy except for NAT so I am sure the router is not being overworked (small site with 3 users) - any ideas of what I can look at ?   Thanks   Rudi