Simple query, exceptionally slow FE performance

I'm really struggling with exceptionally slow performance in Desktop and need help on figuring out where the issue is

It's taking 300ms to calculate one Measure, with 80% of the time in the formula engine (if that makes any difference to the solution). There are 42 million records in the biggest line of the query plan.

It's not a lot of data, and it can't be inefficient DAX that's making it take 5 minutes. Has to be something else but I don't know what else to do.

Where do I look?

This is the SQL for the slowest step as measured by SE timings - I can't see FE timings in DAX Studio

SELECT TOP (1000001) 

[t5].[Created Date],[t5].[Appointment Start],[t5].[Activity ID],[t5].[Adviser Relationship Type],[t5].[Appointment Subject],[t5].[Appointment Description],[t5].[Appointment Status],[t5].[Created By],[t5].[Owner],[t5].[Owner Region],[t5].[Appointment Duration],[t5].[Associated Parties],[t5].[Associated Party Count],[t5].[Organiser Region],[t5].[Adviser CON],[t5].[Adviser Name],[t5].[Booked By],[t5].[Hub],[t5].[Owner Memeber Type],

COUNT_BIG(*) AS [a0]

FROM 
(
(select [activity_created_date] as [Created Date],
    [activity_due_date] as [Appointment Start],
    [activity_id] as [Activity ID],
    [activity_contact_adviser_relationship_type] as [Adviser Relationship Type],
    [activity_subject] as [Appointment Subject],
    [appointment_description] as [Appointment Description],
    [activity_status] as [Appointment Status],
    [activity_createdby] as [Created By],
    [activity_owner] as [Owner],
    [activity_owner_region] as [Owner Region],
    [activity_scheduled_duration_minutes] as [Appointment Duration],
    [activity_associatedparty] as [Associated Parties],
    [activity_associatedcount] as [Associated Party Count],
    [activity_organiser_region] as [Organiser Region],
    [activity_contact_code] as [Adviser CON],
    [activity_contact_name] as [Adviser Name],
    [activity_booked_by] as [Booked By],
    [member_type] as [Member Type],
    [owner_member_type] as [Owner Memeber Type],
    [hub] as [Hub]
from [mart].[sales__appointments] as [$Table])
)
 AS [t5]
WHERE 
(
(
([t5].[Member Type] IN (N'bda',N'ebdm',N''))
)
 AND 
(
1 = 1
)
)

GROUP BY [t5].[Created Date],[t5].[Appointment Start],[t5].[Activity ID],[t5].[Adviser Relationship Type],[t5].[Appointment Subject],[t5].[Appointment Description],[t5].[Appointment Status],[t5].[Created By],[t5].[Owner],[t5].[Owner Region],[t5].[Appointment Duration],[t5].[Associated Parties],[t5].[Associated Party Count],[t5].[Organiser Region],[t5].[Adviser CON],[t5].[Adviser Name],[t5].[Booked By],[t5].[Hub],[t5].[Owner Memeber Type]