SQL LEFT JOIN and SUM() slow - tsql

Question

I'm running a query for calculating the total length of an object.
The object is made out of several smaller segments(from a different table).
Running queries with a single segment for each object works fine, but the load gets really slow for adding more segments. (3000 objects and 9000 segments).
Below is the relevant part of my query:
SELECT
t1.ObjectID,
SUM(SegmentLength) AS TotalLength
FROM Objects t1
LEFT JOIN
Segment t2
ON
t1.ObjectID=t2.ObjectID AND
t1.ProjektID=t2.ProjektID AND
t1.CraneID = t2.CraneID
WHERE
t1.ObjectID='esttest3' AND
(x_ > 0 OR y_ > 0) AND
x_ IS NOT NULL AND
y_ IS NOT NULL
GROUP BY
t1.ObjectID
This takes about 1 minute for 3000 objects with 3 segments each.
I've tried adding indices:
FROM Objects t1 WITH (INDEX(IX_Object_All))
LEFT JOIN
Segment t2 WITH (INDEX(IX_Segment_All))
But this did not improve performance.
EDIT: These indexes were non-clustered, creating clustered indices fixed the issue. (No reference to indices was needed).
If I change the LEFT JOIN into a INNER JOIN and this makes the extracted query(the one above) instantaneous(~0 sec). However inner join still takes a minute in my original query with extra fields (basically making it a left join again I suppose, since there will be values for each object)
I also tried to create a view with calculated TotalLength and making a LEFT JOIN with the view. Still no improvement.
My final idea is to create a trigger for the Segment table to update a TotalLength field in the Object table. I worry however that this will negatively impact performance when importing projects with ~9k segments, updating the value for each one...