Heavy wheels at high speed involve some voodoo physics.
The ideal ride is a heavy frame, with a very light wheel/tire, and the suspension spring is light and progressive, meaning it gets stiffer the more you compress it. Metal coil-springs are nice, but a cylinder with an air-spring works very well.
So you hit a bump. The wheel and tire are forced to change direction and swing upwards. In a hard tail frame, the wheel and frame act together of course, and the rear of the frame is jolted and forced to float in the air for a while.
Having a swingarm and a spring allows the upwards impulse of the wheel/tire to transfer all that sudden energy to the frame in a more spread out time frame, so it's less of a jolt, and more of a gentle push.
If the wheel is light, it responds by moving both up and down easily, while the frame is less affected.
If the wheel is heavier, then...once it gets moving up, it's harder to stop it and reverse direction. If it bottoms out the suspension spring, then at the top of the wheels' upwards travel there will be a jolt as it hits the frame.
You can install a stiffer spring, but then you will feel more of the small bumps that you didnt even feel before, and the big bumps will be even more of a jolt, and less of a gentle push, so...there is a balance each rider has to choose.
If you compare a light bike with a heavy hub, and a heavy bike with a light wheel (mid-mount motor), then...when you both hit the same bump hard, the heavy hub wheel will stay in the air longer. It's bad for the rear wheel, and VERY bad for the front wheel.
If you are in a curve (even a slight curve) and hit a bump, your bike stops traveling on a curved path, and starts traveling in a straight line. When the wheels finally come down and touch the pavement, they suddenly have traction again.
The longer the tire is in the air, the worse the correction will need to be, and you don't even have to gain airspace under the tire for this to affect you. If you hit a bump, and the front or rear wheel comes up just a little, it can still be touching pavement, but lose enough traction to start a skid.
The slower you go, the easier it is to manage. Imagine going into a fast curve, leaning over just a little, and your front or rear tire hits a piece of random debris that acts like a bump. It's not about the comfort of the rider, suspension is about not crashing when you go fast.
And I havent even mentioned spoke issues or gyroscopic issues, but others can cover that better than me.
