Avoid Big Downhills on a Penny Board
Speed wobbles are definitely very common if you're sending it down a hill without knowing what you're getting into. This becomes very real especially for Penny boards because they weren't designed for downhill. They are made to be cheap cruisers you can scoot around your neighborhood on. If you're having issues with speedwoblles, check out a longboard. They will be able to handle faster speeds and hills way easier.
If you're looking for a board you can tackle rolling hills and mini mountains, look towards a longboard as the newest addition to your arsenal.
Longboards are constructed with better materials than plastic Penny's to handle speed and whatever else the road has to throw at you. The various longboard shapes as well will give you more foot room and create a much more comfortable ride for getting around and will give you way more confidence when a hill is thrown at you.
Look For a Board That Favors Stability
Longboards are going to have way more stable trucks and wheels as well compared to Penny's smaller components. The larger wheels on a longboard will be very helpful for going faster and be more stable, but more importantly longboard trucks will give you way more stability with their larger size than those on a Penny.
Get a Board That's Customizable for the Riding You're Doing
Now even on longboards you can still get wobbles if you're not careful but there's a couple ways to get even more locked in. Like those trucks for example, longboard trucks will be way easier to swap out the bushings inside them for better shapes and harder durometers better suited to fit your weight and higher speeds like double barrels. This being way harder to do on Penny board trucks being unable to fit the proper bushings for downhill.
Then you have actual downhill boards, designed to bomb down hills and throw drifts on hairpins with maximum comfort once you learn the ropes. If you're really trying to get serious with your hills, definitely check out getting a board designed specifically for that discipline and experience the adrenaline rush that is Downhill Skating.