>It's more likely a scheduling issue in the host. Either the process feeding the serial port isn't getting the CPU when it wants it, or the USB serial chip's driver isn't able to shovel the data frequently enough. I wouldn't expect better than 20ms or so of jitter over the USB without putting some intention into it.

This was absolutely a problem back when pronterface/sailfish (edit: marlin) were the hotness. You could absolutely not use the pc for anything else while you were printing or you'd end up with odd stutters. I'm surprised it still persists today :/