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I'm using Mathematica 12.0 Student Edition. I'm a little confused by the length of time Mathematica takes to evaluate certain sums of Kronecker Deltas or Discrete Deltas. Here's a simple example below:


My global assumptions are

ClearAll["Global'*"]
$Assumptions = M \[Element] PositiveIntegers;

When I time the double sum of a Kronecker delta below, I get .171875 seconds:

Timing[Sum[KroneckerDelta[i, j], {i, 1, M}, {j, 1, M}]]

When I time a similar sum, I get 14.7656 seconds (!):

Timing[Sum[1/2 (1 + KroneckerDelta[i, j]), {i, 1, M}, {j, 1, M}]]

Why is there a spike in the amount of time Mathematica takes to evaluate this second sum? Could you please give me a general method to speed up the second evaluation?

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