Crab With The Golden Claws: Desert firefight

Hi all. I was reading The Crab With The Golden Claws today and I noticed something rather odd. Tintin and Haddock set off with two guides into the desert to reach the coast. They have been travelling for a long time, at least 8 hours or so, when Lieutenant Delcourt receives a wireless messages alerting him of the raiders along the route that Tintin and co. are taking. The firefight breaks out between Tintin and the raiders and lasts for a short time. I assume it lasts a maximum of ten minutes or so. The reason it is over is because of the Lieutenant and his men.

This makes me wonder how quickly the Lieutenant manages to catch up with Tintin's group? How far away was Tintin from the Wells of Kefheir, which is where he meets the raiders? And how did the lieutenant catch up so quickly (or at least that's how it appears in the book)?

There may have been a long period of time between the scene where Delcourt receives the telegram and the one where Tintin and co are fired at by the raiders.

Also, they may have gotten up late that morning after camping overnight and been travelling at a slow pace since Tintin and Haddock would not be used to riding camels. More experienced riders like Delcourt and his men would have travelled at a faster pace.