I agree the SDL tranny can be slow to respond but I never use the kickdown switch and rarely pull back the shift lever. I don't drive like a granny either. If these cars were absolutely unresponsive, no one would have paid whatever they cost when new.

Make sure the Bowden cable is attached to the throttle linkage. There's and adjustment nut at the engine bay end of the cable. When you pop off the socket to adjust the cable, make sure you feel tension when you tug on the socket. Shortening the cable should make the transmission more responsive including holding low gear longer. If the socket doesn't resist tugging, it might not be attached at the transmission end.

By chance are you running withou the ALDA?

Sixto
87 300D