low perfomances migrating from 9.3 to 9.5

We switched from PostgreSQL server 9.3 to 9.5. From the very beginning there was a noticeable drop in performances (for example : when injecting our SQL dumps into 9.5, COPY and CREATE INDEX were very slow).

Our configuration file was the same for 9.3 and 9.5, except for the "chekpoint segment" that has been deprecated.

I haven't got (yet) some objective performance measurements to show, because the problem was so obvious (just by looking at the log feed) that we switched back quickly to 9.3 (production constraints).