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The part of the code on an ATmega core that does setup() and loop() is at follows:

#include <Arduino.h>

int main(void)
{
        init();

#if defined(USBCON)
        USBDevice.attach();
#endif
        
        setup();
    
        for (;;) {
                loop();
                if (serialEventRun) serialEventRun();
        }
        
        return 0;
}

Pretty simple, but there is the overhead of the serialEventRun(); in there.

Let's compare two simple sketches:

void setup()
{
    
}

volatile uint8_t x;

void loop()
{
    
    x = 1;

}

and

void setup()
{
    
}

volatile uint8_t x;

void loop()
{
    while(true)
    {
        x = 1;
    }
}

The x and volatile is just to ensure it isn't optimised out.

In the ASM produced, you get different results:

You can see the while(true) just performs a rjmp (relative jump) back a few instructions, whereas loop() performs a subtraction, comparison and call. This is 4 instructions vs 1 instruction.