Why can’t we use synchronous counters for high clk frequencies?
Asynchronous counters that cascade edges that are slower (f/2) after each stage and don’t have setup and hold times of sync counters.  Although you’d have to stop the clock and wait for it to ripple thru to read the count, if the prop. Delay exceeded the clock interval.
So pre-scalers used in real-time freq. synthesis are limited to small counts and max frequency must use synchronous counters.