Slow MDX count when expanding query - mdx

Question

Trying to create a calculated measure to be used in a KPI. The measure should count all the calls (servicedesk tickets) that have a runtime larger than 7 days.
Tested the measure with this query, which runs fast (< 3 seconds).
WITH MEMBER [Measures].[Count of Calls long runtime] AS
Count(
Filter(
[Call Details].[Call Number].MEMBERS
, [Measures].[Closed Call Run Time (days)] > 7
)
)
SELECT
{
[Count of Calls long runtime]
} ON 0
FROM
[Business Intelligence]
However when adding other members to the query, suddenly it takes forever to complete:
WITH MEMBER [Measures].[Count of Calls long runtime] AS
Count(
Filter(
(
{
[Ipc Categorisation].[Categorisation].[Subcategory].&[222]
,[Ipc Categorisation].[Categorisation].[Subcategory].&[484]
}
, [Call Details].[Call Number].[Call Number].MEMBERS
)
, [Measures].[Closed Call Run Time (days)] > 7
)
)
SELECT
{
[Count of Calls long runtime]
} ON 0
,
{
[Customer].[Customer].[Customer]
} ON 1
FROM
[Business Intelligence]
WHERE
[Date].[Month Calendar].[Year].&[2016]
There should be about 40 of 30000 calls showing up, divided over 50 customers.
When I change [Count of Calls long runtime] to another measure (calculated or in fact table), the query runs fast.
I would like to understand why this is happening. What can I do to fix this?

Answer 1

FILTER is at best avoided. It forces calculations to run in cell by cell mode. Let's try to change the calculation to run in block computation mode:
WITH MEMBER [Measures].IsGrt7 AS
IIF
(
[Measures].[Closed Call Run Time (days)] > 7,
1,
NULL
)
MEMBER [Measures].[Count of Calls long runtime] AS
SUM
(
{
[Ipc Categorisation].[Categorisation].[Subcategory].&[222]
,[Ipc Categorisation].[Categorisation].[Subcategory].&[484]
} * [Call Details].[Call Number].[Call Number].MEMBERS
,
[Measures].IsGrt7
)
To read more on this subject, see here.

Answer 2

Also worth trying feeding it into a nonempty named set before hitting the caculation:
WITH
SET [CallNonEmpty] AS
NONEMPTY(
{
[Ipc Categorisation].[Categorisation].[Subcategory].&[222]
,[Ipc Categorisation].[Categorisation].[Subcategory].&[484]
}
*[Call Details].[Call Number].[Call Number].MEMBERS
, [Measures].[Closed Call Run Time (days)]
)
MEMBER [Measures].[Count of Calls long runtime] AS
SUM(
EXISTING [CallNonEmpty],
IIF(
[Measures].[Closed Call Run Time (days)] > 7
,1
,NULL
)
)
SELECT
[Count of Calls long runtime] ON 0,
[Customer].[Customer].[Customer] ON 1
FROM [Business Intelligence]
WHERE
[Date].[Month Calendar].[Year].&[2016];