The problem here is that you forgot that asyncio gives you cooperative multitasking. That means you need to hand of control periodically whenever you're doing a

The problem here is that you forgot that asyncio gives you cooperative multitasking. That means you need to hand of control periodically whenever you're doing anything CPU intensive or you end up starving everything else. That happens when you do things like an I/O operation or sleep. Either that, or you hand off the CPU intensive stuff to something on a seperate thread. Now, I'm used to this stuff from doing GUI programming on RISC OS back in the day, and got to know instinctively when I should be using WIMP_PollIdle there. For asyncio at work, I've some wrappers around range(), iter(), enumerate(), &c., that will periodically hand off control with a zero length sleep occasionally in CPU-bound code.