Could she be in pain? Sometimes increased respiration rate can be due to pain. Founder is quite serious and its possible its causing her considerable pain. But even so, a respiration rate of 50 is extremely high for a horse at rest.
When I bestride him, I soar, I am a hawk: he trots the air; the earth sings when he touches it; the basest horn of his hoof is more musical than the pipe of Hermes. ~William Shakespeare