This is the 14061st most frequent Spanish word.
tardar
To take time or to be late.
The word 'tardar' here indicates the amount of time needed by the train to perform its action of arriving.
El tren suele tardar 30 minutos en llegar.
The train usually takes 30 minutes to arrive.
In this sentence, 'tardar' is used to denote the user's aspired minimum time frame required to complete the action of replying to an email.
No quiero tardar mucho en responder a este correo.
I don't want to take long to reply to this email.
Here, 'tardar' is applied to inquire about the estimated time needed by the person to complete the action of finishing the project.