I connect with Arduino 1.8.13 on MacOS Mojave 10.14.6 to an M5Stack Core2 ESP32 device. With the Serial Monitor open, if there is no data out of the M5Stack for a little while and then there is, it takes a while for the Serial Monitor to realise there is data. When it does wake up it displays the data all in one go. Sometimes it's just a second or two, sometimes it's longer (10 seconds?). It always updates immediately if I move the mouse pointer.