Here's a very simple way using sp_spaceused function

declare @tblname nvarchar(100)

Select @tblname = 'yyyyyyyyyy' -- put your table name here

declare @test-2 table(name varchar(100),numrows bigint, reserved nvarchar(1000),data nvarchar(1000),index_size nvarchar(1000),unused nvarchar(1000))

Insert into @test-2 exec sp_spaceused @tblname

Select numrows from @test-2

This gives the results within seconds for even very big tables (>100GB size)