Thanks Yuri, I've tweaked Java a bit according your recommendations, I will report on the results later (right now the production hangs and I'm unable to terminate Java processes :) 

Correct me if I'm wrong but latest IRIS distributions come with Java 11 which rarely requires fine-tuning

Sure, here it is:

Class User.TestService Extends EnsLib.Kafka.Service
{

Method OnProcessInput(pInput As %Net.Remote.Object, Output pOutput As %RegisteredObject) As %Status {
    Set tSC = $$$OK
    Try {
        Set tMsg = ##class(EnsLib.Kafka.Message).%New()
        Do tMsg.FromRemoteObject(pInput)

        #Dim row as Test.Stat
        Set row = ##class(Test.Stat).%New()
        Set row.Topic = tMsg.topic
        Set row.Key = tMsg.key
        // row.Ts - generated automatically in the Test.Stat constructor
        Do row.%Save()
        $$$LOGINFO("Saved")
    }
    Catch (ex) {
        Set tSC = ex.AsStatus()
    }
    Quit tSC
}

}

Hi Peter and thanks a lot for the idea. There is an index for that column. I rebuilt the index and now it is fast. So simple.