They are nowhere near as heavy, because they have a proper API. Not because they arnt doing a shit ton of arithmetic. They are simply not being forced to do it in a backasswards way.
Next time you're in a 40 man battleground while your DPS meter is trying to keep up, you tell me how much math and heavy lifting it isnt doing with no distinguishable framerate loss. Fact is, merely placing some blocks in minecraft will result in framerate loss for no good reason. Try and place 50 or so conduits in a flatworld with vsync off. I am certain those conduits are doing far less than that DPS meter in such a situation. Not even close.
No, that's pathetically easy and simple. All you are doing is taking numbers that are already being crunched by the server and displaying them for the end-user.
LUA script is pathetically lightweight, which is what WoW addons are written in. If you can program ComputerCraft code, you can write WoW addons. It's that simple and easy to do.
Besides, even if it had to calculate mean DPS by itself client-side, that's still a pathetically easy calculation to keep up with. It's just basic math, hell DORVAC could keep up with that. The equation is:
(running tally of damage dealt by player) / (running tally of number of seconds since the calculation first started)
Then you just have a throttle that says 'if it doesn't produce damage in x seconds, reset both running tallies'.
If you are getting fancy, you save that value to a variable to further calculate into raid-long DPS averages, peak DPS during raid, and other such figures.
That's easy.
Base class reflections and adding in new libraries? THAT's a whole magnitude larger. Several magnitudes, on a logarithmic scale.
WoW addons are pathetically easy because WoW comes with a core function that functions... well... rather like ComputerCraft actually. It interprets LUA code in certain ways. It won't let you write code it doesn't like (such as actually changing game mechanics, warping, or other such shenanigans).
In fact, the more I look at it, the better the comparison between WoW addons and ComputerCraft programs are. It's something created by the user running to make his gaming experience better. Written in a lightweight and stripped down version of LUA. And takes almost no CPU usage.