An easier and more efficient way to combine lists?

Question

I currently have a two lists. The first list contains independent variables $x$, and the second list contains dependent variables in the form of $\{\{f[1][x]\},\{f[2][x]\},...,\{f[n][x]\}\}$.

I want to combine them in the form $$\{\{\{x1,f[1][x1]\},\{x2,f[1][x2]\},...,\{xn,f[1][xn]\}\},\{\{x1,f[2][x1]\},\{x2,f[2][x2]\},...,\{xn,f[2][xn]\}\},...,\{\{x1,f[n][x1]\},\{x2,f[n][x2]\},...,\{xn,f[n][xn]\}\}\}$$ ...an easy format for ListPlot.

For some example data:

a = Range[10];
b = a^2;
c = (a + 1/2)^2;
fa = {b,c};

Now one can easily do this with Table:

Table[{a[[j]], fa[[i, j]]}, {i, Length[fa]}, {j, Length[c]}]

but knowing Mathematica's many functions I thought there might be an easier way. I tried this as well:

Transpose@MapThread[Tuples@{{#1}, #2} &, {a, Transpose@fa}]

but with the multiple Transpose calls, I figured there would be a slight performance hit. And there was (2.854 vs 3.261 seconds for vectors with 1MM elements on my machine).

Is there an easier and more efficient way to combine these lists?

Answer 1

Thread[{a, #}] & /@ fa

or

Inner[List, a, #, List] & /@ fa