Sql-server – AWS RDS query running extremely slowly

I am testing out AWS RDS to replace a local DB and in general I am finding the latency etc acceptable for most of our use-cases. There is one particular query that we run manually (i.e. in SQL Server Management Studio) from time to time that takes a while to execute. With the DB as it stands, that query is taking just under 2 minutes to run on my local machine. However, I left the same query running on an identical DB on RDS running for well over 4 hours and it still had not completed! (it actually runs so long that I have no idea if it ever completes).

The query itself only returns 23 records, 5 columns each. This is a tiny amount of data that needs to be sent back so I can't see this being a network latency issue. I am still testing this out on the free RDS (i.e. lowest spec) but I'm hardly running it on a supercomputer locally so I could understand it taking 10, even 50 times longer but over 4 hours doesn't seem right just because the server is the lowest spec offered.

Also note that the DB table being queried has the same index applied etc.

So I am hoping that there is something about my query maybe that AWS RDS is particularly bad at, or requires some config change first? The query does use a scalar-valued user defined function which uses TSQL TOP.

I'll post the query code here, I don't expect anyone to analyse it though, I was more hoping someone knows of a config setting on RDS or a particular feature of TSQL to avoid?

Query:

DECLARE @N as int = 1,                                      
        @Ticker as nvarchar(50) = 'USDZAR',                                     
        @StartDate as date = '2015-06-08'                                       
SELECT DISTINCT M.TradeDate,                                        
                T.[Moneyness-],                                     
                T.[Volatility-],                                        
                T.[Moneyness+],                                     
                T.[Volatility+]                                     
FROM YieldX_MTM as M                                        
LEFT JOIN (SELECT C.TradeDate, C.Moneyness as [Moneyness-], C.Volatility as [Volatility-],                                      
             D.Moneyness as [Moneyness+], D.Volatility as [Volatility+]                                 
       FROM (SELECT TradeDate, MAX(Strike/Future) as Moneyness, MIN(Volatility) as Volatility                                   
        FROM YieldX_MTM                             
        WHERE TradeDate > @StartDate                                
          AND Ticker = @Ticker                              
          AND Category = 'Foreign Exchange Future'                              
          AND InstrumentType <> 'F'                             
          AND Strike/Future <= 1                                
          AND Expiry = dbo.GetRelativeExpiryCurrency(TradeDate, @N, @Ticker)                                
        GROUP BY TradeDate) as C                                

       JOIN (SELECT TradeDate, MIN(Strike/Future) as Moneyness, MIN(Volatility) as Volatility                                   
        FROM YieldX_MTM                             
        WHERE TradeDate > @StartDate                                
          AND Ticker = @Ticker                              
          AND Category = 'Foreign Exchange Future'                              
          AND InstrumentType <> 'F'                             
          AND Strike/Future >= 1                                
          AND Expiry = dbo.GetRelativeExpiryCurrency(TradeDate, @N, @Ticker)                                
        GROUP BY TradeDate) as D                                
       ON C.TradeDate = D.TradeDate) as T                                   
  ON M.TradeDate = T.TradeDate                                      
WHERE M.TradeDate > @StartDate                                      
  AND Ticker = @Ticker                                      
  AND Category = 'Foreign Exchange Future'                                      
  AND InstrumentType <> 'F'                                     
  AND Expiry = dbo.GetRelativeExpiryCurrency(M.TradeDate, @N, @Ticker)                                      
ORDER BY TradeDate  

GetRelativeExpiryCurrency UDF:

CREATE FUNCTION [dbo].[GetRelativeExpiryCurrency]
(
    @Date DATE,
    @N INT, 
    @Ticker NVARCHAR(50)
)
RETURNS DATE
AS
BEGIN
    -- Declare the return variable here
    DECLARE @Expiry as DATE;

    IF @N > 0
    BEGIN
        SELECT @Expiry = MAX(Expiry)
        FROM (SELECT DISTINCT TOP(@N) Expiry
              FROM YieldX_MTM
              WHERE Expiry >= @Date
                AND Ticker = @Ticker
                AND Category = 'Foreign Exchange Future'
              ORDER BY Expiry) as T
    END
    ELSE
    BEGIN
        SELECT @Expiry = MIN(Expiry)
        FROM (SELECT DISTINCT TOP(-@N) Expiry
              FROM YieldX_MTM
              WHERE Expiry < @Date
                AND Ticker = @Ticker
                AND Category = 'Foreign Exchange Future'
              ORDER BY Expiry DESC) as T
    END
    RETURN @Expiry

END