That’s what I’d expected. I’m using an even lower CPU clock in an FreeRTOS application and also handle a USART and other peripherals reliably.
This brings us back to the question what’s going on here 
Strange…
NVIC prio grouping is disabled, too ? Using NVIC_SetPriorityGrouping(0);
BTW besides the good FreeRTOS documentation there is a good article here