If you made a fast descent, it’s probably because the controllers requested that the pilot "expedite descent through X,000 feet for traffic." It's no big deal; they just do it to avoid traffic conflicts. Controllers have a bubble that they have to keep around each aircraft on their scopes. Computers can predict some traffic conflicts, but mostly it’s up to the controller to predict such events. I assure you, there was nothing unsafe or even odd about what happened to you. In the military, we get that all the time.
I'll start with the easy answers. Airspeed is dependent on airframe. Different airframes have different limiting airspeeds, and whatever airspeed you descend at is normally up to the pilot. I flew a small business jet and our max speed was 330 knots. I'm not an airline pilot, so I imagine they fly as fast as possible until they reached 10,000 feet above sea level. In which case, the speed limit is 250 nautical miles per hour (or knots). Within the terminal airspace of a major airport, that max airspeed decreases to 200 knots. Of course, there are exceptions for aircraft that need to fly faster to maintain a safe operating airspeed, like fighter jets. Descents are normally made using an instrument called a Vertical Velocity Indicator. It measures velocity, up and down, in feet per minuet.
Angle of descent is also another one that varies. Three degrees is normally what an aircraft uses on final approach to a runway and for the most part, it provides for a nice, gentle, comfortable descent in any situation. Now for how far away to begin a descent. There’s a lot of background math that goes into and I'll skip the derivation and go right to the formula we use. It's:
Height to lose divided by descent gradient equals distance from airfield to begin descent.
For height to lose, you take the altitude you're currently at (in your case, 30,000 feet) minus the altitude that approach control wants you at to begin your final approach, (ballpark, its normally around 2000 feet) and for descent gradient, 3 degrees gives you 300 feet loss for every mile of horizontal distance covered. So you get 28,000 divided by 300. The pilot would need to begin their descent about 94 miles from the airfield for a comfortable, 3 degree descent.
Now, an easy method is to just take the altitude you're at (30,000), drop off the last three zeros (30) and multiply by 3. This method gives you 90 miles to begin your descent. It's less accurate, but works in a pinch until you can figure it out with more precision. Normally, a pilot will work that formula backwards during the descent to find out if they need to increase the rate of descent to meet a restriction. Of course, there are no penalties for arriving at a lower altitude early, but the longer you can stay in a descent, the more gas you save and more comfortable the ride. Hope this helps!