if count or match after lookup very slow

aggregate query like below very slow [{
"$lookup": {
"from": "a" ,
"let":

{ "item": "b" }

,
"pipeline": [
{
"$match": {
"$expr": {
"$and":[

{"$eq": [ "$_id", "$$item" ]}

]
}
}
},

{ "$limit":1 }

],
"as": "tmp_item"
}
},

{ "$count":"count"}

]
I must put $count or $match after $lookup because of use lookup's results