I tried something like this previously while working on Playlink Pong. I found that the issue with using serial in this way wasn't the speed per se, but the fact that playdate has trouble receiving serial messages while it is sending serial messages.
Any time I wrote a function that printed too often, it would result in the serial callback running sporadically, meaning that the program would not receive data regularly enough. It's possible that this is totally normal for how "serial" connections function, but as a newbie this surprised me. Isn't the whole point of 'serial' that the devices broadcast one after another?
I assume there is some way of syncing up the playdate and PC components to overcome this problem, but it was above my head.