The tunnel is quick; the queue to reach it isn't
Atal Tunnel south portal, Manali, Himachal Pradesh
The Atal Tunnel turns the drive towards Lahaul into a short hop, which is why so many day-trippers head that way at once. On busy weekends the slow part is the road from Manali to the south portal, not the tunnel itself.
The lesson
Leave Manali early, before the day-trip traffic builds.