I fly allot at speed, often in open areas and it does a pretty good job at staying low if I tell it to but as soon as it detects objects extending above it's height floor it gains altitude. It makes the decision pretty far out which I like because that means it doesn't slow down and handles it smoothly.
There's 2 issues to consider, if it's staying low and there's trees around it's not going to keep up, it will slow then rise up to avoid the obstacle and continue on. For me that means slowing to let it catch up. The other issue is when I've forced it to fly lower then it wanted in those conditions at speed is the only times I've crashed it. The faster it flies the further it's looking out. If I'm in the open desert it will stay pretty close to the 8' height floor. I've learned not to turn that off at the speeds I use the SD2, bad things happen.