performance tuning - Thread over list in different levels

I have these two lists, both with the same length in the first dimension:

l1={{a1,a2},{b1,b2},{c1,c2}};
l2={{{1,2,3},{4,5,6}},{{10,11,12},{13,14,15},{16,17,18}},{{19,20,21}}};

And I need mix both to get into this result:

{{a1,a2,1,2,3},{a1,a2,4,5,6},{b1,b2,10,11,12},{b1,b2,13,14,15},
{b1,b2,16,17,18},{c1,c2,19,20,21}}

I have done this code that do the job:

Flatten[MapThread[Function[{a,b},Join[a,#]&/@b][#1,#2]&,{l1,l2}],1]

But I think it could be simpler. Some clue?

Answer

I don't know why no one thought to use ArrayFlatten:

wizard1[l1_, l2_] := ArrayFlatten @ MapThread[Append, {l1, l2}]

wizard2[l1_, l2_] := ArrayFlatten @ Join[l1, List /@ l2, 2]

Test:

wizard1[l1, l2]

wizard1[l1, l2] === wizard2[l1, l2]

 {{a1, a2, 1, 2, 3}, {a1, a2, 4, 5, 6}, {b1, b2, 10, 11, 12},
  {b1, b2, 13, 14, 15}, {b1, b2, 16, 17, 18}, {c1, c2, 19, 20, 21}}

True

According to ssch's test suite these functions are also the fastest:

testSpeed[50000, 15]

enter image description here

Comments

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