I took mine off the other day and noticed it flies and turns faster but is more unstable in strong wind (Tello can't handle wind coming up from underneath it). Oh it's quieter too.
from my personal experience, it affects the flying time a little bit about 1 min (not so sure).
but to me, I use Tello in a light wind condition for taking short drone videos or maybe just playing around. If you're flying it in high-windy place, you'll never gonna think about taking of the propeller since it could be blown if it's too light-weighted.
Prop-guard is a bit better attached, you know keeping the weight down a bit.