I have this DAX formula that gives me a count of id that appears on the fact table in a month, averaged over the year. I can put this measure is a table ad it's unpacked by row with no issues (by adding variables from dimensions)

Measure:= AVERAGEX(

SUMMARIZE(

CALCULATETABLE(fact_table;FILTER('Time_Dimension';'Time_Dimension'[Last_month] <> "LAST"));

Time_Dimension[Month Name];

"Count";DISTINCTCOUNT(fact_table[ID])

);

[Count]

)

But it's terrible slow (I have 3 measures like this on a single table) and the fact table is big (like 300Million rows big)

I was reading that SUMMARIZE perform really bad with aggregations and It should be replaced with SUMMARIZECOLUMNS. I wrote this formula

Measure_v2:= AVERAGEX(

SUMMARIZECOLUMNS(

Time_Dimension[Month Name];

FILTER(Time_Dimension;

Time_Dimension[Month Name]<>"LAST"

);

"Count";DISTINCTCOUNT(fact_table[ID])

)

[Count]

)

And it works when I visualize the measure as it is, but when I try to put it in a context (like the table above) it gives me the error "Can't use SUMMARIZECOLUMN and ADDMISSINGITEMS() in this context" How can I make a sustainable optimization from the original SUMMARIZE function?