Unrealistic values of same tests after repeated executions

The following simple tests behave differently in a second execution.

using BenchmarkTools
using LinearAlgebra
a = rand(300,300); u = triu(rand(300,300));
@btime u*a;      # multiplication without exploiting upper triangular shape
@btime UpperTriangular(u)*a;     # multiplication with exploitation of upper triangular shape
@btime lmul!(UpperTriangular(u),a);  # in-place multiplication
@btime u*a;      # multiplication without exploiting upper triangular shape
@btime UpperTriangular(u)*a;     # multiplication with exploitation of upper triangular shape
@btime lmul!(UpperTriangular(u),a);  # in-place multiplication

These are the results folowing a new start of Julia (other executions produce different figures):

julia> @btime u*a;      # multiplication without exploiting upper triangular shape
  281.900 μs (2 allocations: 703.17 KiB)

julia> @btime UpperTriangular(u)*a;     # multiplication with exploitation of upper triangular shape
  167.200 μs (3 allocations: 703.19 KiB)

julia> @btime lmul!(UpperTriangular(u),view(a,:,:));  # in-place multiplication
  845.800 μs (5 allocations: 176 bytes)

julia> @btime u*a;      # multiplication without exploiting upper triangular shape
  3.432 ms (2 allocations: 703.17 KiB)

julia> @btime UpperTriangular(u)*a;     # multiplication with exploitation of upper triangular shape
  3.011 ms (3 allocations: 703.19 KiB)

julia> @btime lmul!(UpperTriangular(u),view(a,:,:));  # in-place multiplication
  3.045 ms (5 allocations: 176 bytes)

Some values show a more than 10-times slow down. Is any explanation for this behaviour?

julia> versioninfo()
Julia Version 1.8.3
Commit 0434deb161 (2022-11-14 20:14 UTC)
Platform Info:
  OS: Windows (x86_64-w64-mingw32)
  CPU: 16 × Intel(R) Core(TM) i7-10700 CPU @ 2.90GHz
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-13.0.1 (ORCJIT, skylake)
  Threads: 1 on 16 virtual cores

Reproduce the reported benchmark sequence from the issue on Julia 1.8.3 under Windows, including the repeated calls to u*a, UpperTriangular(u)*a, and lmul!. Compare the first and second results, then determine and document the cause of the reported slowdown; done means the differing timings have a confirmed explanation or a narrowly defined fix.