This is the 3601st most frequent Japanese word.
遅い
Slow; late.
Here, "遅い" is used to describe the train's movement speed, indicating it is slow.
電車が遅い。
The train is slow.
In this sentence, "遅い" describes the timeliness of the action, indicating it is overdue.
宿題を提出するのが遅い。
Submitting homework is late.
"遅い" in this context is used to describe the speed of his reaction, implying it is not prompt.