Look in the cam's PTZ/Control tab in BI.
there's a setting for 'speed' at the lower right that may do what you need...
You might also try OXML as the PTZ protocol (instead of DLink).
not sure if that would make a difference (or even work,) but I use OXML
for my PTZ's (Huisun/Imporx) and the speed of pan/tilt is inversely
proportional to the zoom level (slower at max zoom, faster at widest view.)
Dunno if it's the cam doing that or BI via OXML, but may be worth a try...