Slow Response when converging from primary to ISDN DBU

[ccokini]

Router is set up to use ISDN as DBU. When primary frame-relay link goes down, there is a good 5-10 second delay with the convergence to dbu....this should only be minimal. Any suggestions why this would happen.
Spoke router 1720
Catcher(hub router) AS5300

2 spids off of 1720. Also having issue where 2nd spid is getting a fast busy...not sure if related..2nd ship does come up and two channels are bonded.