Hello

I did a small benchmark with 8 `$let`

made the query **4x slower**.

Seems that every nested `$let`

, we add makes the query **50% slower**, for example i tested

also with 2 nested `$let`

and query took double time, which is very big difference.

```
[0 ... 1000] array
map 1 function, that adds 7 times the number 1
[7 8 .... 1007]
```

The first time i used

`(+ .. (+ (+ n 1) 1))`

7x

The second time i used

`n1=n+ 1, n2=n1+1, n3=n2+1`

7x in a nested `$let`

```
{
"$let": {"vars": {"n1": {"$add": ["$$n",1]}},
"in": { "$let": {"vars": {"n2": {"$add": [ "$$n1",1]}} ... 7x
```

Times were

```
728msec $let
180msec $add
Query became 4x slower.
```

I tried exactly the same in a programming language and i got zero difference, nested let didn’t cost anything.

Why `$let`

in MQL costs so much, and is there a way to ask for it to be fixed?

Using variables with `$let`

is very important, escpecially in data processing.

Thank you