How to improve Performance for this Dax

Hi vengadesh_p  Try this:

ActiveContracts =
VAR StartofThisPeriod = LASTDATE(Dim_Calendar[Month Start Date])
VAR EndofThisPeriod = ENDOFMONTH(StartofThisPeriod)
RETURN
CALCULATE(
    [# Contracts],
    KEEPFILTERS(
        Fact_EwContract[Contracts Start Month] <= StartofThisPeriod &&
        Fact_EwContract[Contracts End Month] > EndofThisPeriod
    )
)

Hope this helps!!

If this solved your problem, please accept it as a solution!!

Best Regards,
Shahariar Hafiz

Hi vengadesh_p 
The very obvious reason causing a performance problem here is that you are filtering a table when you only need to filter a column or columns. shafiz_p has provided a good example on how to apply a filter over a column but still keep  the row context.  You can also refer this blog: https://www.sqlbi.com/articles/using-keepfilters-in-dax/