Slow starter is a myth and a crutch. What does it even mean because there is no basis in science for it to be true. Maybe the issue is the guy is a bit of a head case?
Baseball is a culture of voodoo, silly habits and black cats running under ladders and something like "slow starter" just feeds and fosters that nonsense
Clearly you haven't looked at his splits....
Mar/April - .225/.318/.387(659 PAs) 96 wRC+
May - .245/.346/.391(639 PAs) 105 wRC+
Jun - .277/.349/.444(714 PAs) 119 wRC+
July - .269/.351/.403(672 PAs) 109 wRC+
Aug - .294/.356/.482(673 PAs) 129 wRC+
Sept/Oct - .258/.355/.375(664 PAs) 105 wRC+
Rationalize it however you want but those are the results. He's hit .276/.359/.427 in the second half over his career and .251/.336/.406 in the first half largely because he's been horrid in April. That 96 wRC+ is 21% worse than is career 2nd half numbers.