Can someone help me diagnose a strange stored procedure performance issue please?

I have a stored procedure (posted below) that returns message recommendations based upon the Yammer Networks you have selected. If I choose one network this query takes less than one second. If I choose another this query takes 9 - 12 seconds.

/****** Object:  StoredProcedure [dbo].[MessageView_GetOutOfContextRecommendations_LargeSet]    Script Date: 2/18/2015 3:10:35 PM ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE PROCEDURE [dbo].[MessageView_GetOutOfContextRecommendations_LargeSet]

	-- Parameters
	@UserID int,
	@SourceMessageID int = 0

AS
BEGIN

	-- variable for @HomeNeworkUserID
	Declare @HomeNeworkUserID int

	-- Set the HomeNetworkID
	Set @HomeNeworkUserID = (Select HomeNetworkUserID From NetworkUser Where UserID = @UserID)

    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON

   -- Begin Select Statement
   Select Top 40 [CreatedDate],[FileDownloadUrl],[HasLinkOrAttachment],[ImagePreviewUrl],[LikesCount],[LinkFileName],[LinkType],[MessageID],[MessageSource],[MessageText],[MessageWebUrl],[NetworkID],[NetworkName],[PosterEmailAddress],[PosterFirstName],[PosterImageUrl],[PosterName],[PosterUserName],[PosterWebUrl],[RepliesCount],[Score],[SmallIconUrl],[Subjects],[SubjectsCount],[UserID]

    -- From View
    From [MessageView]

     -- Do Not Return Any Messages That Have Been Recommended To This User Already
    Where [MessageID] Not In (Select MessageID From MessageRecommendationHistory 
        Where UserID = @UserID)

    -- Do Not Return Any Messages Created By This User
    And [UserID] != @UserID

    -- Do Not Return The MessageID
    And [MessageID] != @SourceMessageID

    -- Only return messages for the Networks the user has selected
    And [NetworkID] In (Select NetworkID From NetworkUser Where 
       [HomeNetworkUserID] = @HomeNeworkUserID And [AllowRecommendations] = 1)

    -- Order By [MessageScore] and [MessageCreatedDate] in reverse order
    Order By [Score] desc, [CreatedDate] desc

END
The Actual Execution Plan Shows up the same; there are more messages on 
the Network that is slow, 2800 versus 1,500 but the difference is ten 
times longer on the slow network.
Is the fact I am doing a Top 40 what makes it slow? 
My first guess was to take the Order By Off and that didn't 
seem to make any difference.
The execution plan is below, it takes 62% of the query to look up the
IX_Message.Score which is the clustered index, so I thought this would 
be fast. Also the Clustered Index Seek for the User.UserID take 26%
which seems high for what it is doing.

I have indexes on every field that is queried on so I am kind of at a loss as to where to go next.

It just seems strange because it is the same view being queried in both cases.

I tried to run the SQL Server Tuning Wizard but it doesn't run on Azure SQL, and my problem doesn't occur on the data in my local database.

Thanks for any guidance, I know a lot of the slowness is due to the lower tier Azure SQL we are using, many of the performance issues weren't noticed when were on the full SQL Server, but the other networks work extremely fast so it has to be something to with having more rows.

In case you need the SQL for the View that I am querying it is:

SET QUOTED_IDENTIFIER ON
GO

CREATE VIEW [dbo].[MessageView]
AS
SELECT M.UserID, M.MessageID, M.NetworkID, N.Name AS NetworkName, M.Subjects, M.SubjectsCount, M.RepliesCount, M.LikesCount, M.CreatedDate, M.MessageText, M.HasLinkOrAttachment, M.Score, M.WebUrl AS MessageWebUrl, U.UserName AS PosterUserName,
        U.Name AS PosterName, U.FirstName AS PosterFirstName, U.ImageUrl AS PosterImageUrl, U.EmailAddress AS PosterEmailAddress, U.WebUrl AS PosterWebUrl, M.MessageSource, M.ImagePreviewUrl, M.LinkFileName, M.FileDownloadUrl, M.LinkType, M.SmallIconUrl
FROM  dbo.Message AS M INNER JOIN
        dbo.Network AS N ON M.NetworkID = N.NetworkID INNER JOIN
        dbo.[User] AS U ON M.UserID = U.UserID

GO

Corby