Substituting elements of array is very slow

General Usage
11

Thanks. Here is the complete code I am running. It is actually just the LinearAlgebra and the BenchmarkTools missing

using LinearAlgebra
using BenchmarkTools


function testfun02(M)
    nrow, ncol = size(M);
    spidx = Array{Float64, 2}(undef, nrow, 3)
    numit = 1
    
    for j01 = 1:ncol, j02 = 1:nrow
        aval = M[j02,j01]*(j01+j02)
        if aval > 0
           spidx[numit,1] = j01
           spidx[numit,2] = j02
           spidx[numit,3] = aval
            numit += 1
        end
    end
    return spidx 
end


B = Array{Float64,2}(I, 10000, 10000);
benchout = @benchmark testfun02(B);
@time testfun02(B);
@time testfun02(B);

The Output is

@time testfun02(B);
  0.111525 seconds (27.23 k allocations: 1.895 MiB, 17.53% compilation time)

@time testfun02(B);
  0.096936 seconds (2 allocations: 234.453 KiB)

The version info is

Julia Version 1.6.2
Commit 1b93d53fc4 (2021-07-14 15:36 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: Intel(R) Core(TM) i7-7700HQ CPU @ 2.80GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-11.0.1 (ORCJIT, skylake)
Environment:
  JULIA_EDITOR = code
  JULIA_NUM_THREADS =

and that looks indeed strange.