Timer misses interrupts if clock ticks per second is set too high

For some reason, CLKTICKS_PER_SEC values that are roughly 90 or larger cause the Pi to not service timer interrupts anymore. Xinu has this value at 1000 by default. I noticed similar behavior in my testing environment when I was trying to do too much in my timer ISR (like synchronously printing on the UART), so I suspect it's because the timer ISR (and possibly the VIC emulator) is/are too slow.
