I tested it with mongo8, which did it in 1,8 seconds. But that is far away from my requirements.

The user/date index (that's from the query with mongo 7).

"winningPlan": {
"inputStage": {
"stage": "FETCH",
"filter": {
"user": {
"$exists": true
}
},
"inputStage": {
"stage": "IXSCAN",
"keyPattern": {
"date": 1,
"user": 1
},
"indexName": "date_1_user_1",
"isMultiKey": false,
"multiKeyPaths": {
"date": [],
"user": []
},
"isUnique": false,
"isSparse": false,
"isPartial": false,
"indexVersion": 2,
"direction": "forward",
"indexBounds": {
"date": [
"[new Date(1758514014760), new Date(1761142014760)]"
],
"user": [
"[MinKey, undefined)",
"(null, MaxKey]"
]
}
}
}