Performance degradation when going on 7.1.1

Hi,

I was previously running 7.1.0 SP2, and upgraded to 7.1.1 on a 4 nodes cluster
Prior to this upgrade, I was running regular queries every 15mn, the database was loaded continuously with new data, projections had been created manually in order to optimize query execution.
The execution times for these queries was relatively stable.
Right after the upgrade to 7.1.1 and restarting the database, I observed an increase in the execution time of most queries, of almost 60% (from 250 sec to 400 sec).
Is there any reason that could explain this ?
I want to be clear: the only thing I did was: stop DB, update vertica, restart db, strictly no change in incoming data flow or config or data model.