New GCP E24 shots with pressurized basket flowing too fast

New owner of GCP E24. My shots flowing too fast (8 seconds) for pressurized baskets and for store bought pre-ground espresso Lavazza. I am not new to espresso just new to this machine. I try to pre-infuse using the steam release hack for a few seconds and then shut it off. I use pressurized basket with black diverter/tiny black plug in place. Coffee just flows like water after first initial drops. I tamp hard and I am certified experience tamper ;-). I am currently working on the aligning the burrs of Shardor 64mm grinder and it is taking a whole week like any other project and hence I have to live with store bought pre-ground coffee with the new Gaggia. I know this has been asked many time but other folks were not using the pressurized baskets. If you use pressurized baskets and pre-ground coffee then how long your shots take? Is the simulated pre-infusion breaking puck and create channeling due to sudden increase in pressure after steam want is shut off? Or is this usual?