Performance issue in Azure

Hi, a customer is running into performance issues on Azure.
His environment: 3 nodes cluster, 8 GB RAM each. 1 index (around 10 fields), 4 GB storage for this index, so very small.
Here are his observations:
A query in dev tools/console takes about 1 sec to complete, first run. Second run can be very fast (10ms) or very slow (again 1 sec).
Same query in search profiler will always run very fast.
After a while (one day later), the same query will run slow again.
The same query on AWS always (!) ran very fast.
Here is a performance dashboard screenshot:

Here is the query:

GET _search
{
  "sort": [
    {
      "timestamp": {
        "order": "desc"
      }
    }
  ],
  "profile": "true",
  "query": {
    "bool": {
      "filter": {
        "bool": {
          "must": [
            {
              "term": {
                "organizationId": "560eb72b-b96e-48a0-a193-02612c069570"
              }
            },
            {
              "terms": {
                "deviceId": [
                  "863723ae-67d9-43ca-8ed1-781bbd3710b7"
                ]
              }
            },
            {
              "range": {
                "timestamp": {
                  "gte": "2021-07-14T12:52:01.044Z"
                }
              }
            }
          ]
        }
      }
    }
  }
}

Is this normal behaviour?