MSSQL paging: the fastest row_number () over paging Stored Procedure for querying the first few pages

Code

  Create     Procedure     [  DBO  ]  .  [  Getpagerrowover  ]   
  @ Pageindex    Int     =     1  ,
  @ Pagesize     Int     =     10  ,
  @ Tables     Nvarchar (  1000  )  =     Null  ,
  @ Fields     Nvarchar  (  2000  )  =     '  * '  ,
  @ Where     Nvarchar  (  2000  )  =     Null  ,
  @ Groupby     Nvarchar  (  2000 )  =     Null  ,
  @ Orderby     Nvarchar  (  1000  )  =     Null  ,
  @ Returncount    Int     =     0  Output
  As  
  Begin  
  Set  Nocount  On  ;
  Declare     @ SQL    Nvarchar  (  4000  ),  @ Startrowindex     Int  ,  @ Maximumrows     Int  ;
  Declare     @ Strwhere    Nvarchar  (  2000  )
  Declare     @ Strgroupby     Nvarchar  (  2000  )

  Set     @ Startrowindex     = (  @ Pageindex     -     1  )  *     @ Pagesize  ;
  Set     @ Maximumrows     =    @ Pagesize  ;

  If     @ Where     Is     Not     Null     And     @ Where     ! =    ''  
  Begin  
  Set     @ Strwhere     =     '  Where  '     +     @ Where  
 End  
  Else  
  Begin  
  Set     @ Strwhere     =     ''  
  End  
  If    @ Groupby     Is     Not     Null     And     @ Groupby     ! =     ''  
  Begin  
 Set     @ Strgroupby     =     '  Group  '     +     @ Groupby  
  End  
  Else  
 Begin  
  Set     @ Strgroupby     =     ''  
  End  

  Set     @ SQL     =    '  Select  '     +     @ Fields     +     '  From (select  '     +     @ Fields    +     '  , Row_number () over (order  '     +     @ Orderby     +     '  ) As rownum from  '     +    @ Tables     +     @ Strwhere     +     @ Strgroupby     +     '  ) As tmptable where rownum>  '     +    Convert  (  Nvarchar  (  10  ),  @ Startrowindex  )  +     '  And rownum <= (  '     +     Convert (  Nvarchar  (  10  ),  @ Startrowindex  )  +     '  +  '     +     Convert  (  Nvarchar (  10  ),  @ Maximumrows  )  +     '  )  '  
  Exec  (  @ SQL  )
  Print     @ SQL 
  Set     @ SQL     =     '  Select @ returncount = count (*) from  '     +     @ Tables     +     @ Strwhere 
  Exec  Sp_executesql  @ SQL  , N  '  @ Returncount int output  '  ,  @ Returncount  Output
  Print     @ SQL  
  End