Sql-server – SQL Azure “hot” table query slowdown over time

I have an Azure SQL Database table that is acting as a "prioritized queue". As such, it gets heavily written to and deleted from over time. What's happening is that after a week or two, the DB size increases. CPU increases and performance reading from this slows down almost to a halt.

If I check the count, it's zero. If I check the table size, there are a lot of pages assigned to this table.

If I TRUNCATE the table, the count is still zero but the table size is back to minimal levels and performance is acceptable again.

Obviously, to me this indicates that the table has a lot of pages due to the high insert/delete activity and this is why the DB size increases. This is fine as when Azure cleanup occurs (presumably transaction log backups), the DB size drops back to normal levels.

What I don't understand is why the performance drops off so drastically when there are still zero rows in the table.

Can someone provide some suggestions as to what may be going on?

NB: The logic dictates that the overall write is atomic; i.e. update business data and write to the queue. Additional logic dictates that if an item (identified via business key) exists then overwrite queue item.

The Table is defined thus:

CREATE TABLE [dbo].[PrioritizedQueue](
    [Id] [bigint] IDENTITY(1,1) NOT NULL,
    [Region] varchar(10) NOT NULL,
    [ItemId] varchar(10) NOT NULL,
    [Priority] tinyint NOT NULL,
    [ReceivedTimestamp] datetime2(7) NOT NULL,
    [Operation] varchar(7168) NOT NULL,
    [Content] varchar(max) NOT NULL,
 CONSTRAINT [PK_IndexerQueue] PRIMARY KEY NONCLUSTERED 
(
    [Id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON)
)
GO
CREATE UNIQUE INDEX [IX_PrioritizedQueue_ItemId] ON [dbo].[PrioritizedQueue]
(
    [ItemId] ASC,
    [Region] ASC
)
INCLUDE ([Priority])
WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF)
GO
CREATE CLUSTERED INDEX [IX_IndexerQueue_Priority] ON [dbo].[PrioritizedQueue]
(
    [Region] ASC,
    [Priority] ASC,
    [ReceivedTimestamp] ASC
)
WITH (STATISTICS_NORECOMPUTE = OFF, DROP_EXISTING = OFF, ONLINE = OFF)
GO

FYI: The "content" field is a varchar(max) due to history, it now only contains a file pointer so AFAIK the row should never need to use data pages off the current page

The Stored proc that reads from the table, and appears to have all the problems is:

CREATE PROCEDURE [dbo].[usp_indexer_dequeue]
    @batchSize AS int,
    @region varchar(10)
AS
BEGIN
    SET NOCOUNT ON;
    SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
    DECLARE @deletedbatch table
    (
        [Operation] varchar(7168) NOT NULL,
        [Content] varchar(max) NOT NULL,
        [ItemId] varchar(10) NOT NULL
    );
    WITH cte AS 
        (
            SELECT TOP(@batchSize) [Operation], [Content], [ItemId]
            FROM [PrioritizedQueue] WITH (rowlock, updlock, readpast) 
            WHERE [Region] = @region
            ORDER BY [Priority], [ReceivedTimestamp] ASC
        )
    DELETE FROM cte
    OUTPUT deleted.*
    INTO @deletedbatch;
    SELECT * FROM @deletedbatch
END
GO

This proc is designed to fetch the top available N rows (typically this 25 rows) return them to the app and then delete from the table. As it runs as one of many processes calling this at once, it uses (rowlock, updlock, readpast) to bypass anything currently locked.