I have just tested vbextreme/Scheduler and Arduino-Scheduler using the following code:

#include <Scheduler.h> 

void setup() {
  pinMode(3, OUTPUT);
  Scheduler.startLoop(loop2);
}

void loop() {
  digitalWrite(3, LOW);
  yield();
  digitalWrite(3, HIGH);
  yield();
}

void loop2() {
  yield();
}

Without any Scheduler lines, the code occupies 734 bytes of ROM, 9 bytes of RAM, and the pin toggles with a period of 8.5 us (117 kHz). The numbers below represent the additional ROM/RAM/execution time:

• vbextreme/Scheduler occupies additional 1676 bytes of ROM, 70 bytes of RAM, and the context switch takes 15.5 us (toggle frequency 14.18 kHz). It seems to only support AVR/SAMD boards.

• vbextreme/Scheduler with CONFIG_AVR_OPTIMIZE_COOPERATIVE set to 1 occupies additional 1568 bytes of ROM, 70 bytes of RAM, and the context switch takes 11 us (toggle frequency 19.07 kHz).

• Arduino-Scheduler occupies additional 578 bytes of ROM, 44 bytes of RAM, and the context switch takes 10.6 us (toggle frequency 19.59 kHz). It supports AVR/SAMD and some of the Teensy/STM32 boards. It seems that this scheduler is not compatible with tasks which use malloc().

• CopyThreads occupies an additional 2148 bytes of ROM, 41 bytes of RAM, and the context switch takes 54.5 us (toggle frequency 4.417 kHz). This scheduler claims to have the advantage of using a single stack, so as long as tasks do not yield during peak stack usage, each task has essentially the complete stack at its disposal.

I couldn't get the equivalent code for TaskScheduler to run, I'll be grateful if someone posts an answer featuring schedulers I didn't test.