fastcos, fastsine, fasttan etc. definition and tips

Hello!
Recently I discovered [fastcos] operator for [gen~]. I wanted to learn more about it (how it works etc) but the docs are not that much revealing.

I was wondering if one can build a cheap-but-decent fm synth based on the [fastcos] rather than [cycle].

[cycle] has pretty short wavetable anyway and would be avoided in a more professional application. It is still antialiased though.

One can then use [cos] with [phasor], but it turns out to be much more cpu expensive.

Maybe [fastcos] could be a golden mean for that approach?

There is also gen~ code made by Ernest (published recently here: https://cycling74.com/forums/synthcore-v2-4-gen~-library-available-for-free-download ):
qcos(x){
    x *= x;
    x = x*(x*(x*(x*(x *-.0000002605 +.000024709
        )- .00138884)+.416667)- .499923)+ 1;
    return x;
}
and as far as I know it is an approximation based on the Taylor series. How is it different from [fastcos]?