And I just know that someone is going to pick at the term "timely" but that simply means that things get done when they need to. If you need uS then you need uS, if you need weeks then that can be accomodated too. Fast really has little to do with it except that when you define the specs for your system you should know how fast you need to respond to the various stimulti. When shopping for an RTOS you can use that as your benchmark. They all specify context switch time (how much time OS takes to save context, evaluate which task should run next and then retore context). You need to apply the numbers the RTOS vendor gives you to your system specifically considering your own processor clock speed (or MIPs) and in some cases C compiler.