Speed up DAX calculation when iterating through each row and a unique identifier

[WorkHard]

I'm trying to calculate the number of times the Start Date has changed for an event.
This works great but it already takes a good 5-7 seconds to calculate 5k rows and I'm anticipating much more rows soon.

-EventId-	-Start Date-	-Snapshot-	-Current Result-
Event1	1/15/2021	1/1/2021	0
Event1	3/15/2021	2/1/2021	1
Event1	3/17/2021	3/1/2021	2
Event1	3/17/2021	4/1/2021	2
Event2	3/1/2021	1/1/2021	0
Event2	4/4/2021	2/1/2021	1
Event2	4/1/2021	3/1/2021	2

c_# of start date changes = 
VAR CurrentEvent= datasource[EventId]
VAR CurrentSnapshot = datasource[Snapshot]
RETURN
CALCULATE(
    DISTINCTCOUNT(datasource[Start Date]),
        FILTER(
            ALL(datasource),
            datasource[EventId]=CurrentEvent
            && datasource[Snapshot] <= Snapshot
        )
)-1

I know there's a way to speed this up using an M calculation but I'm not familiar with them.

This table is already combined from multiple files (it uses a folder data source) so I'm not even sure I can use M in this scenario.

[amitchandak]

@WorkHard , I think you are trying the right way. remove all from the column.  Or Try a  measure. Give code for that too

c_# of start date changes = 
CALCULATE(
    DISTINCTCOUNT(datasource[Start Date]),
        FILTER(
            (datasource),
            datasource[EventId]=earlier(datasource[EventId])
            && datasource[Snapshot] <= earlier(datasource[Snapshot])
        )
)-1

or measure 

c_# of start date changes = 
CALCULATE(
    DISTINCTCOUNT(datasource[Start Date]),
        FILTER(
            ALL(datasource),
            datasource[EventId]=max(datasource[EventId])
            && datasource[Snapshot] <= max(datasource[Snapshot])
        )
)-1

[WorkHard]

Thanks but removing the variables before the calculate method can only go one snapshot earlier. The "earlier" function can only detect 1 snapshot back this way. There are events that change the start date 5 times in the span of 5 snapshots.

The measure will not work in my case because the results need to be static, no matter how the report is filtered.