Really. CPF publishes an Annual Report with lots of facts and figures. The 2016 edition is available
here. To start your calculation journey, take a look at
table 13 on page 92: "Government Grants to Members." Do some careful math, then report back to us what CPF's actual return is.
Somebody else did that math recently for CPF LIFE (separately) and determined that that leg of CPF offers a 7.1% net effective nominal yield, which is damn good of course. Anyway, see what you can figure out, and let us know....
....And then, "So what?" You're apparently trying to argue that CPF ought to have a net effective yield comparable to the government's sovereign wealth fund. Why? That's bonkers from a public policy point of view, unless you believe Singapore and future Singaporeans don't have much of a future?