Fast insert from temp table to main table - advice needed, match index order?
I think that you may need to look at redesigning the clustered index on your main table.  Best practices for a clustered index is to make it unique and ever increasing, but you have the first column being sensorid.  Which means that (assuming I have inferred correctly) that you have a relatively stable set of sensors with lots of date/time and measurements.  

This will lead to lots of page forwards and an extremely fragmented, which increases your maintenance duration as your index defragmentation process works.  Without knowing your actual query load, I can only make broad suggestions.


Change your clustered index to be Date/Time, SensorID, pkID
Add a non-clustered index on SensorID, Date/Time


With that in place, your inserts won't fragment the clustered index as badly.  BulkInserts won't cause blocking to read workloads.  Having a clustered index on the stage table that matches it's target will help.