I'd have to agree with what Neal said re: people having 1/2 crosses and then going onto 3/4 bred's to get more pace, but in doing so losing to much of the collie attribute's that they liked in the 1/2 bred dog.
Seen some "very" racey 3/4 bred's over the year's, even owned one myself many moon's ago that threw so racey I could of ran her at the local flapping track no problem.
9 out of 10 half bred dog's will have more than enough pace for rabbit's, daytime or lamped