Performance problem with measure

Hi all,

I am trying to get a measure working to get information about the average cumulative duration of sorted data. 

The following two tables are used:

Log:

Cases:

Expected result in a matrix:

This works fine with the following when DurationAtStart is the following measure until I work with a big dataset:

DurationAtStart = 
VAR auxT_ =
    ADDCOLUMNS (
        Log,
        "ValueSum_",
            CALCULATE (
                SUM ( Log[duration] ),
                Log[event_number] <= EARLIER ( Log[event_number] ) - 1,
                ALLEXCEPT ( Log, Log[id] )
            )
    )
RETURN
    AVERAGEX ( auxT_, [ValueSum_] )

Do you any tips on how I can change the measure so everything works faster? Right now on a bigger dataset it stops calculating because the memory limit will get reached. I don't want to pre calculate everything unless it really is the last option.

Thank you for your help!

Lucmax