Willie Mays -

1951 after 12 games .163 .294 .326 .620
1952 after 15 games .205 .314 .330 .643
1954 after 19 games .244 .268 .423 .691
1956 after 22 games .209 .303 .384 .687

There are a lot more examples of slow starts just from this guy, but the point is that his OPS over those seasons was .966.