Why is this closure over keyword arguments causing such a dramatic slowdown?

General Usage Performance
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7

Following up on that with a test, deepcopy doesn’t seem to help me here:

function L0(arg1; kwarg1, kwarg2, kwarg3, kwarg4)
    arg1 + kwarg1 + kwarg2 + kwarg3 + kwarg4
end

function freeze_params(func_to_freeze, params_to_freeze)
    local_params_to_freeze = deepcopy(params_to_freeze)
    return (arg1; kwargs...) -> func_to_freeze(arg1; kwargs..., local_params_to_freeze...)
end

allkwargs = (kwarg1=1, kwarg2=2, kwarg3=3, kwarg4=4)
firstkwargs = (kwarg1=1, kwarg2=2)
lastkwargs = (kwarg3=3, kwarg4=4)

L1(arg1; kwargs...) = freeze_params(L0, firstkwargs)

@btime L0(0; $allkwargs...) # 1.205 ns (0 allocations: 0 bytes)
@btime L1(0; $lastkwargs...) # 73.484 ns (3 allocations: 400 bytes)