Many ways to create a short delay In Unity, coroutines, increment in an Update loop, using Invoke with a delay
Which method is the LEAST resource heavy (ie for mobile)?
I honestly don’t think there’s a huge difference. Choose the option that fits your needs and is easiest to implement in the situation.
Personally I lean to certain choices, depending on what I need.
If for some reason, you’re finding a performance problem, investigate to determine the cause and try to see if you can re-organize or restructure what’s wrong at that point in time. 
Barring doing something crazy or with a LOT of objects, performance won’t be a concern for how you do the delay process.
HOWEVER, I highly recommend that you choose the way that you can most-easily explain to get your delay, and this might be different in each case you need such a thing. The reason you want to be able to easily explain it is because if one month from now as you are wrapping up your game you find that there is a bug in the delay, you want to be confident in understanding precisely how this delay was implemented. The very fact that delayed activities take place over multiple rendered frames makes them harder to debug, and therefore you want to keep it nice and safe and simple and straightforward, and in this case, performance problems from this type of code are very unlikely.
Good info.
Yes I’m finding it necessary to take detailed notes as I work on more complex apps. I forget what I was doing yesterday let alone several months ago:)