Just for the record my dad owned a 318 back in the early days of that model and he had a 2.5 acre yard to mow. Trust me I cut that yard as fast as possible and the 318 was always able to cut well at speeds that were probably about 5-6 mph I would guess. I know it seemed like I was flying to mow with that JD. It had a lot of power for a two rotor deck which allowed it to keep the speed on the blades up and as long as the blades were sharp it would cut great. It still took a good while to mow that much grass. He bought a 400 series after that which had 3 blades and I always thought it took longer to mow with that machine than with the 318.
My 324 will cut pretty well at top speed but I only have a few areas in my yard where I can get it going that fast. I mow about an acre. I do know the 324 cuts my grass much faster than my LX172 does. I've used that mower for 17 years and it's been a great mower but I can cut the yard in about half the time with the 324. My only problem is the quick turns start to give me motion sickness after a while and I don't get motion sickness that easy. I haven't had it in probably 30 years in fact but I've been getting it on the 324 because it will turn very quickly if it's on level ground.
My 324 will cut pretty well at top speed but I only have a few areas in my yard where I can get it going that fast. I mow about an acre. I do know the 324 cuts my grass much faster than my LX172 does. I've used that mower for 17 years and it's been a great mower but I can cut the yard in about half the time with the 324. My only problem is the quick turns start to give me motion sickness after a while and I don't get motion sickness that easy. I haven't had it in probably 30 years in fact but I've been getting it on the 324 because it will turn very quickly if it's on level ground.