From the very moment Tanjiro was introduced, he performed much better than anyone could have realistically assumed when Giyuu provoked him. Tanjiro, while emotionally overwhelmed and panicked, managed to faint a simplistic rage attack while actually planning a clever ruse by throwing his axe behind a tree hoping it would take Giyuu down after he was stuck. May I remind you all that this is a 13 year old boy with no prior fighting experience?

Tanjiro, with his back against the wall, gravely injured, thinking Inosuke and Tengen were dead and left alone with two very powerful demons, managed to create a plan in barely a few seconds to a minute that got him closer to beheading Gyutaro than Tengen ever was. (Despite Tanjiro being quite weaker than Tengen and Gyutaro in EDA.) 

He managed to quickly remember useful information about Thunder Breathing given to him by Zenitsu while he was in a dire situation and successfully assimilated the basics of Thunder Breathing into his own style as a way to compensate for his injured foot. 

He made a very good observation about how Akaza’s hits seemed to be drawn to his vitals like magnets and began breaking down his Blood Demon Art and developing an effective counter to it pretty quickly all things considered. He even made a very good deduction reminding himself of his accidental usage of the Transparent World during his fight with Hantengu. 

And I could probably go on glazing my goat for longer but you get the point. 😅

Tanjiro consistently has some of the best Battle IQ showings in the manga and doesn’t get enough praise for it amongst fans in my opinion