Note that you can have your loop() function act like a traditional "main()" and not actually loop (never "return")
The Arduino "core" that calls loop IS:
int main(void)
{
init();
setup();
for (;;)
loop();
return 0;
}
So it's not doing anything for each loop to slow down ... anything. For all practical purpose, the only code that takes time to run is YOUR code.