Does awaiting a non-Promise have any detectable effect?

await is not a no-op. If the awaited thing is not a promise, it is wrapped in a promise, that promise is awaited. Therefore await changes the execution order (but you should not rely on it nevertheless):

console.log(1);
(async function() {
  var x = await 5; // remove await to see 1,3,2
  console.log(3);
})();
console.log(2);

Additionally await does not only work on instanceof Promises but on every object with a .then method:

await { then(cb) { /* nowhere */ } };
console.log("will never happen");

Is there any detectable effect of awaiting a non-Promise?

Sure, .then gets called if it exists on the awaited thing.

Is there any difference in behavior one should be aware of to avoid a potential error?

Don't name a method "then" if you don't want it to be a Promise.

Any performance differences?

Sure, if you await things you will always defer the continuation to a microtask. But as always: You won't probably notice it (as a human observing the outcome).

Completely agreed with Jonas's statements. One thing that was not answered in his question was Are the following two lines completely same or do they theoretically differ?:

following two lines are not completely same, they're theoretically different.

1. var x = 5
2. var x = await 5

execution time in my console for 1st and 2nd statement is 0.008056640625ms and 0.055908203125ms respectively. async/await, setTimeOut etc are APIs provided by Run time in which JavaScript Run time is running. Putting await on a non-promise will be executed in event-loop. Line 1 will be executed right after reaching the stack but the line 2 will take few time(milliseconds) as it will 1st go to the stack and then to the task queue after skipping webAPI waiting section because there's no promise to be resolved & finally after that control will be given to stack again for execution.