I looked at Baseball Savant. Alcantara has already showed elite bat speed. Based on the very small sample (5 competitive swings) he'd rank 13th in MLB between Juan Soto and Vlad Jr in bat speed among qualified hitters. He's above De La Cruz, Alonso, Soler, Tatis Jr, Witt Jr. Has the same bat speed as Mike Trout. Not sure if his numbers may drop with more swings, this stat measures avg bat speed on competitive swings, not max bat speed.
On the Cubs he ranks above Wisdom and below Morel and Canario (who has insane bat speed). He has the longest swing on the Cubs per Savant which isn't surprising. Gotta think the HR power will come for him, and given the bat speed and swing length he may whiff a lot.
PCA to compare shows well below-average bat speed and avg to below-average max exit velo.