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Airline Pilot -fag here, anons. ADSB is not required for the majority of aircraft until 2020. At that point you will need ADSB to fly in Class A,B,C and E Airpace above 10000', but not above 2500' AGL.
The airlines have adopted ADSB features for safety and economy reasons. Corporate operators as well. The military has ADSB, but are under no order to use it for public consumption. If they do not want you to see them, you will not see them. They are like Navy Seals when they want to be- operations occur and you'll never know about them until years have passed. Maybe never.
ADSB tracks GROUNDSPEED. Any airliner or corporate/military type business jet cruises in the mid 30's to low 40's in altitude. Newer Gulfstream, Canadair, Falcon type aircraft routinely cruise in the upper 40's. Groundspeeds in the 420- 520 knot range are normal and common. Airliners cruise at ,76- .79 mach. (76%-79% of the speed of sound). Some newer corporate jets are most efficient at mid to upper 40's altitude, and are capable of .83-.88 mach while there.
Groundspeed does not denote being in a hurry or carrying "top brass". If top brass or someone else is being taken somewhere- you won't know about it until its over.
Airlinepilotfag- over and out.
Speed of sound is 1100 fps. Depending upon altitude/distance- there will always be a delay between when an aircraft flies past and you hear it. Also- keep in mind the newer high bypass engines are designed such that the flow of hot exhaust gasses are shrouded inside a flow of colder ambient air. This blocks much of the noise making for quieter more neighborhood friendly jets. Older generation of turbojet engines were incredibly good at turning fuel into noise, which I loved, but they were very good at creating angry airport neighbors. Hope this helps.