Adding `Subtract` definition affects performance of BoolEval function

Question

Due to some bug shown in Differences of Quantities gives different result in different Mathematica version, I add an additional definition to Subtract. But I found it will severely slow down Szabolcs' BoolEval function.

A fresh Mathematica kernel, run

array = RandomReal[1, 1000000];
Pick[array, ResourceFunction["BoolEval"][array > 0.5], 
   1]; // RepeatedTiming

gives

{0.0163395, Null}

Another fresh Mathematica kernal, run

Unprotect[Subtract];
Subtract[x_Quantity, y_Quantity] := x - y;
Protect[Subtract];
array = RandomReal[1, 1000000];
Pick[array, ResourceFunction["BoolEval"][array > 0.5], 
   1]; // RepeatedTiming

gives

{0.521434, Null}

Why does adding an additional definition affect so much? What is more, how to fix Subtract quantity bug without affecting BoolEval?

update

Now I know how to fix Subtract quantity bug without affecting BoolEval. In the BoolEval package, replace all Subtract[a,b] and similar with a-b, then it will not affected by new definition of Subtract. But I still do not know why adding an additional Subtract definition affect so much?