Can you paste the exact aggregation you used the first time? I understand the second test, but not quite the first one, where you say
(+ .. (+ (+ n 1) 1))7x
Although I just ran a test with your second pipeline and 8 levels of $let and I cannot get things to be slower than 1ms so clearly something is different in our tests. Could you provide both complete pipelines and describe the data that is in your test collection?