Question for Demon Slayer fans, how real is the breathing?

Like is the water/fire Tanjiro uses real? And can Zen move at lightning speeds?

I don't think so but some people do.

It can be really hard to tell sometimes, but no. It's just a visual display usually, they don't actually produce the related elements. Strongest proof of this being red blades, which can be caused by changing the blade's color red. Tanjiro needed Nezuko's blood to do this the first time, which he wouldn't if sun breathing actually produced fire. Beyond that the elements don't play a physical role. Zenitsu does produce audible thunderclaps which Tanjiro has heard. Take that as you will, some think those are sonic booms.

The only breath that definitely produces physical phenomenon is Moon Breathing, possibly as a demon power, as Kokushibou's sword comes from his body and is possibly his blood demon art. Wind Breathing might produced ranged wind attacks, it's super ambiguous. At first glance, it definitely looks like it does, but when you look really closely at the instances in question, it's not 100% clear that's what Sanemi is doing, and no one comments that he does so.

Think there's arguable evidence for both. At the beginning of the series, it's quite clear that the techniques are just techniques (water-breathing for example), but as the series goes on, it becomes more ambiguous, fights begin to have this more supernatural, mystical feel to it. Breathing techniques are still the same, but it seems amped to the point you aren't really sure that they're just simple techniques anymore. The examples of Wind-breathing, Moon-breathing for example.

But I think Sun-breathing is the best example.

well in the anime it definatly real

there the old lady rengoku father saved

and in here memory we see her flames

that enough proof for the elements to be real in the anime

They aren't real according to Gotouge herself.

Still better than your favourite shounen though

They aren't the real element but they do actually exist