Because stall speed is torque-based,

any time you add a larger turbo (especially a fixed geometry) without swapping to a higher stall converter, your stall speed will drop. When this happens, drivability suffers. Retarding your injection timing can help bring the charger to life quicker (or adjusting vane positioning if you went with a larger VGT), but there is only so much your tuner can do to make an improperly-spec’d converter work with a larger turbo.