Karna's prana consumption Throughout the fate universe, there are many strong servants. And one of them is Karna, who is said by Nasu to be on par with Gilgamesh.
But it is a fact that he consumes so much mana that an ordinary master can't handle it. A basic mage can't even keep his mana burst(flames) activated continuously for 10 seconds. Why does this happen?
The reason I'm asking this is simple. We have Gilgamesh, who uses thousands of noble phantasms (and that is his main offence) and still doesn't drain his master. This one may not seem like a good example (since he uses hate of Babylon) so let's look at another case;
Artoria(saber). She was summoned in fuyuki ALIVE. She is different from other servants this way. Since she was summoned like this and not in a servant container as how it should be, she had no choice but to use mana burst to match in strength with other servants. And even though she was using mana burst again and again, Shirou could handle it.
Why is it like this? Does this mean that Karna alone is like this? Will his prana consumption decrease if he summoned in archer or rider class?
Gilgamesh is not using hundreds of NPs. He's using the Gate of Babylon, just opening a portal, shooting a projectile and then retrieving it, it's actually very cost effective. The only weapons he can actually release their True Names are Ea and Enkidu.
Source from Fate Complete Material III, on Gilgamesh: Tactics

Gilgamesh battles by shooting the originals of all Noble Phantasms like bullets via Gate of Babylon. The tactic yields excellent destructive power to magic energy efficiency. Since every Noble Phantasm is a bullet, the real magical energy used is merely the activation cost of Gate of Babylon. Because he possesses the originals of all Noble Phantasms, he can assault the weakness of any Servants. As a result, Gilgamesh has the advantage over most of the Servants in combat.

Artoria doesn't even have a proper path with Shirou, she's using her own prana reserves to use her skills. So no, Shirou is not sustaining her Mana Bursts.
Karna's Mana Burst (Flame) is just very high-maintenance. Extremely powerful as well, the dude is a fucking demigod, easily Heracles tier, who also has terrible prana consumption, Illya is probably the only one who could sustain Karna, much like she did with Herc. This last bit is pure speculation.