Does impact increase w/run speed?

I fully believe there is a sweet spot in a person's run speed. It increases as you get more fit and faster. I actually find running really slow rougher on my joints than hard efforts

Speed is not what causes the injuries. It's the effort.

From your narrative, you mentioned that you ramped up mileage quickly, were running at a higher effort level, and were not paying attention to what your body was doing, and you developed shin splints. It was impact from run speed that caused your shin splints. It was the fact that you ramped up mileage quickly, maintained a higher level of effort, and ignored your body.

Impact may or may not increase with speed; I don't know, I'm not a scientist. But I do know that it is a red herring in this case. It wasn't how fast you ran that created the problem. It was the fact that you kept running hard while increasing mileage quickly, and probably to some extent sloppy form. If you take things easy, you will let your body develop its own ability to handle the stresses, and thus it doesn't matter one way or the other.

Proper running technique helps as well, especially for shin splits, knee and hip problems. I bet that you increased your distance too rapidly and focusing on form allowed you to run without putting strain on your body.

With proper running technique impact from running is almost a non issue...

In addition to what others have said, as you increase intensity form can start to break down. You may be reaching more with your foot, trying to artificially lengthen your stride. This reaching will increase the stress on your shins as you land harder on your heel. As you increase speed, try your best to maintain form. When your form breaks down, back off a little.