mjhjmtu

I have a few large arrays (100 times larger than in the example) and I need to re-arrange them. Basically using tests on b and c, I need to pick elements of a.

a = RandomReal[1, 10^7];
b = RandomReal[1, 10^7];
c = RandomReal[1, 10^7];

sel = MapThread[(Abs[#1] <= 0.1) ∧ (Abs[#2] > 0.9)&, b, c]; // AbsoluteTiming

10.9262,Null

d = Pick[a, sel]; // AbsoluteTiming

0.584831,Null

MapThread and Pick are the only ways to do this, as far as I found, I can't re-arrange the code any more.
The selection generation takes awhile, many times longer than on other platforms, is there a faster way to speed this up?

You can use UnitStep to construct your selector array:

SeedRandom[1]
a = RandomReal[1, 10^7];
b = RandomReal[1, 10^7];
c = RandomReal[1, 10^7];

sel = MapThread[(Abs[#1] <= 0.1) ∧ (Abs[#2] > 0.9) &, b, c]; //
 AbsoluteTiming // First

15.332532

d = Pick[a, sel]; // AbsoluteTiming // First

0.605384

sel2 = UnitStep[.1 - Abs[b]] UnitStep[Abs[c] - .9]; // AbsoluteTiming // First

0.439621

d2 = Pick[a, sel2, 1]; // AbsoluteTiming // First

0.031240

d == d2

True

If sel is already created, you can convert it a packed array. Additional processing time to do this is more than compensated for by speed gains in Pick:

sel3 = Developer`ToPackedArray[With[True = 1, False = 0, Evaluate[sel]]]; // 
 AbsoluteTiming // First

0.37475

d3 = Pick[a, sel3, 1]; // AbsoluteTiming // First

0.031245

d == d3

True

See this answer by Mr.Wizard for the origin of the With[True = 1, False = 0, ...] trick.