Seemingly equivalent queries take vastly different execution times
SELECT Data.timestamp, Labels.label, Data.value 
FROM Data 
JOIN DataItems 
      on DataItems.dataItemId = Data.dataItemId 
     and Data.assetId = 134
     AND Data.timestamp between '2016-09-27T12:00:00Z' AND '2016-09-27T14:00:00Z'
JOIN Labels 
      on DataItems.labelId = Labels.labelId
     AND Label.label IN ('Eng_RPM','Eng_Load','Eng_OilTemp')


maybe add a non-clustered index on Label.label