Just curious what size tiller are you running ? I have a 40HP with a 72" and never had had it bog down or even lower the RPMs even when breaking new ground, I did have to dial the slip clutch in but it never bogged the tractor down, I would make shallow passes at first then worked each pass a little deeper. our clay is like concrete when it has no moisture so I try to till after a rain if possible
5 foot. I am tilling 4 inches or so deep. We have a good bit of clay. Also tractor is only 24 hp.