precedence of setInterval, eval and print execution

While the interpreter is idle, it checks for intervals, timeouts, and other events, and runs the associated handlers. After such a callback returns, interpreter is back in idle state (until the next callback fires).

So, if you have something slow-ish, that will block callbacks from firing until it's done. The Espruino will queue up callbacks during this time, but that may not be good enough, depending on your application.

This really points to a fundamental difference between what Arduino and Espruino are good at. Espruino makes it so easy to interact with the world via http/etc, write civilized, modern code in a forgiving language, gives you a live interpreter, and generally incredible ease of coding. Anything where you are doing a lot with strings is way easier. But the price of that is that it's pretty slow - there are plenty of things I can do on an 8mhz AVR that the 72mhz stm32 or 160mhz ESP8266 running Espruino just isn't fast enough for, and similarly, things where sub-second timing is critical are often troublesome. I have a few applications where I use an Espruino for the stuff that handles strings, talks to the world, and so on, talking to an AVR dedicated to the timing critical stuff (in my case, dealing with 433mhz OOK RF) over serial.....

For more detailed assessment, post the code you're observing this with and what board (and espruino version) you're using it on, and an example of what it's receiving over serial (the content of the string could be relevant); I find it hard to belive that what you've described would end up messing the timing up by hundreds of ms - but who knows what you're doing when processing the received string, or how your serial handler is written, and so on...