SELECT *
FROM TableA A
  JOIN Table B ON A.ObjectID = B.ObjectID AND A.Time = B.Time
WHERE ...

SELECT *
FROM TableA A
  JOIN TableB B ON B.ID IN
  (
      SELECT TOP 1 ID
      FROM TableB
      WHERE ObjectID = A.ObjectID AND Time >= A.Time
  )
WHERE A.ObjectID = 'b6bea656-b1ff-46c2-be17-dfb13ab5c2a1' AND A.Time >= 06/27/2012 07:50:36'; 

CREATE INDEX IDX_TableA_ObjectID ON TableA (ObjectID); CREATE INDEX IDX_TableA_Time ON TableA (Time);
CREATE INDEX IDX_TableB_ObjectID ON TableB (ObjectID); CREATE INDEX IDX_TableB_Time ON TableB (Time);

SELECT *
FROM TableA A
  JOIN TableB B ON B.ID IN
  (
      SELECT TOP 1 B2.ID
      FROM TableB B2
      WHERE B2.ObjectID = A.ObjectID AND B2.Time >= A.Time
  )
WHERE A.ObjectID = 'b6bea656-b1ff-46c2-be17-dfb13ab5c2a1' AND A.Time >= 06/27/2012 07:50:36';