[Duugu]

local _G = _G 
function test()
    local start = time()
    for x = 1, 200000000 do
        local a = _G["TargetFrame"]
        
    end
    print(time() - start)
end

Result: 8

function test()
    local start = time()
    for x = 1, 200000000 do
        local a = _G["TargetFrame"]
        
    end
    print(time() - start)
end

Result: 10

But can't say why.

[edit]
Keep in mind that are 200,000,000 loops. In words: two hundred million. The difference is ~2 secs. 0.00000001 secs per access.
Guess it's save to ignore that in most cases.