This interesting point deserves amplification.
Tractor tires being both large and lightly inflated, they act as springs. So the whole tractor's mass is sprung weight, excepting the tire tread area. True, there's no formal shock-absorbing system, but there is quite a bit of loss in sidewall flexing and air heating. The tire always returns to net shape, but a lot of energy is lost in the process. In one kind of language, there is nil geometric hysteresis, but a relatively low coefficient of restitution - how high the ball bounces, so to speak.
The issue with not having a car-type suspension shows up with repeated bumps, as in driving fast on rough ground. Even without the perfect resonance that corrugated roads can produce (and that feeds back to deepen the corrugation), bounces can add up or cancel, and we notice the ones that add up. More sprung mass means more stored energy, so the bounces can be harder, especially since since hitting the next bump before returning to "neutral" means compressing a stiffer spring. At the same time, the increased mass can damp motion: a ten-ton machine will be shaken less by any given bump. So it may not always be a simple analysis.
Driving just a little faster sometimes takes the vehicle out of the resonance regime, if you feel safe trying that. Adding well-fixed weight front and rear increases both the total mass and the pitching moment of inertia, and a little might help a lot. With my previous tractor, I found that a BH in back and full loader bucket in front actually let me drive smoothly at much higher speeds over rough ground. Another reason to have an FEL even on a tractor mainly used for mowing?