The optimization process and recommendations:

1. Avoid Correlated Subqueries (query line: 10): A correlated subquery is a subquery that contains a reference (column: EventId) to a table that also appears in the outer query. Usually correlated queries can be rewritten with a join clause, which is the best practice. The database optimizer handles joins much better than correlated subqueries. Therefore, rephrasing the query with a join will allow the optimizer to use the most efficient execution plan for the query.
2. Avoid Subqueries (query line: 24): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, it's recommended to join a newly created temporary table that holds the data, which also includes the relevant search index.
3. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
4. Mixed Order By Directions Prevents Index Use (query line: 41): The database will not use a sorting index (if exists) in cases where the query mixes ASC (the default if not specified) and DESC order. To avoid filesort, you may consider using the same order type for all columns. Another option that will allow you to switch one direction to another is to create a new reversed "sort" column (max_sort - sort) and index it instead.
5. Prefer Direct Join Over Joined Subquery (query line: 29): We advise against using subqueries as they are not optimized well by the optimizer. Therefore, we recommend to replace subqueries with JOIN clauses.

Optimal indexes for this query:

CREATE INDEX events_idx_deviceid_timeend ON dbo.Events (DeviceId,TimeEnd);

The optimized query:

SELECT
        [Project2].[EventId] AS [EventId],
        [Project2].[DeviceId] AS [DeviceId],
        [Project2].[TimeEnd] AS [TimeEnd],
        [Project2].[C2] AS [C1],
        [Project2].[EventId1] AS [EventId1],
        [Project2].[Name] AS [Name],
        [Project2].[C1] AS [C2] 
    FROM
        (SELECT
            [Limit1].[EventId] AS [EventId],
            [Limit1].[TimeEnd] AS [TimeEnd],
            [Limit1].[DeviceId] AS [DeviceId],
            [Extent2].[Name] AS [Name],
            [Extent2].[EventId] AS [EventId1],
            CASE 
                WHEN ([Extent2].[Value] IS NULL) THEN CAST(NULL AS float) 
                ELSE ROUND([Extent2].[Value],
                1) END AS [C1],
CASE 
    WHEN ([Extent2].[Value] IS NULL) THEN CAST(NULL AS int) 
    ELSE 1 END AS [C2] 
FROM
(SELECT
    [Project1].[EventId] AS [EventId],
    [Project1].[TimeEnd] AS [TimeEnd],
    [Project1].[DeviceId] AS [DeviceId] 
FROM
    [dbo].[Events] AS [Project1] 
WHERE
    (
        [Project1].[DeviceId] = 1
    ) 
ORDER BY
    [Project1].[TimeEnd] DESC) AS [Limit1] 
LEFT OUTER JOIN
[dbo].[Octaves] AS [Extent2] 
    ON [Limit1].[EventId] = [Extent2].[EventId]
) AS [Project2] 
ORDER BY
[Project2].[TimeEnd] DESC,
[Project2].[EventId] ASC,
[Project2].[C2] ASC