Bolony. Complex highly stressed cast iron components are run at thousands of RPM every day. Look at standard transmission flywheels and pressure plates for example. Consider autoengnes, turbine rotors, and other high speed apparatus.
Just because somethng looks a little scary doesn't mean it actually is. Don't get me wrong, out of balance workpieces, defective chucks etc pose their hazards if run at high RPM. And workpieces dislodged from the chuck from wrecks or high risk operation have caused their share of injuries too. You have to be sensible about thngs like this. Work a little engineering math, consider structural safety, clamping forces being self relieved from centrifugal force ect.
The mechanical strength of chuck body materials can be ranked in descending order for assesment. Heat heated forged alloy steel counter-centrifugal production chucks naturally are at the top of the list followed by cast alloy steel, semi-steel, solid body cast iron, cored cast iron and old worn-out chucks. Three jaw chucks resist the effect of centrifugal force better than four jaws.
I've seen 10 ft vertical boring mill tables run at 30 RPM (truely frightening) and a 26 ft vertical boring mill table run at 4 RPM with the table extension outrggers deployed (brave men run at that sight.) It aint really scary unless you plan to dive in it.
Some machine are chuck speed limited because of the power loss from jaw windage or wear and tear on the clutch from accellerating a large mass to high RPM. It aint that the chuck's gonna blow up; they're worried the clutch might fry.
Friable frangible vitrious bonded grinding wheels run safely at 6000 ft per minute and may burst at 14,000. Even a miserable quality import cast iron chuck can hold together at 6000 feet per min with the jaws firmly gripping a balanced work piece. So let that be your limitation. 6000 feet per min on a 12" chuck is 1910 RPM. Problem is windage. The jaws are like a squirrel cage fan. They can eat up 3 or 4 HP blowing air around running that fast. If your lathe has only 5 HP there's not much left over for cuttin'.